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A.  TEXT-BOOK  OF  GEOLOGY 


Text-Book  of  Geology 

PUBLISHED  BY 

John  Wiley  &  Sons,  Inc. 


PART  I 

Physical  Geology.  By  the  late  L.  V.  Pirsson.  Second,  Re- 
vised Edition;  vii  +  470  pages,  6  by  9;  317  figures  in 
text;  appendix,  index  and  folding  colored  Geological 
map  of  North  America.  Cloth,  $3.00  net. 

PART  II 

Historical  Geology.  By  Charles  Schuchert.  Second  Edi- 
tion, Rewritten  and  Enlarged;  viii  +  724  pages;  6  by  9; 
237  figures  in  text;  47  plates;  index  and  folding  colored 
geological  map  of  North  America.  Cloth,  $4.50  net. 


Frontispiece 

Barohoini  Natural  Bridge  (Piute  for  rainbow);  northwest  of  Navajo  Moun- 
tain, southern  Utah.  Work  of  erosion  in  LaPlata  Sandstone.  Height  309 
feet;  width  between  abutments  278  feet;  causeway  at  top  33  feet  wide. 

(Photo  by  H.  E.  Gregory.) 


A  TEXT-BOOK  OF 

GEOLOGY 


FOR  USE  IN  UNIVERSITIES,  COLLEGES,  SCHOOLS  OF 
SCIENCE,    ETC.,  AND    FOR    THE    GENERAL    READER 


PART  I— PHYSICAL  GEOLOGY 


BY 


LOUIS  V.   PIRSSON 


LATE  PROFESSOR  OF  PHYSICAL  GEOLOGY  IN  THE  SHEFFIELD  SCIENTIFIC  SCHOOL 
OF  YALE  UNIVERSITY 


PART  II— HISTORICAL  GEOLOGY 


BY 

CHARLES  SCHUCHERT 

PROFESSOR  EMERITUS  OF  PALEONTOLOGY  IN  YALE  UNIVERSITY  AND  OP  HISTORICAL 
GEOLOGY  IN  THE  SHEFFIELD  SCIENTIFIC  SCHOOL  OF  YALE  UNIVERSITY 


PART  I 


SECOND,  REVISED  EDITION 


NEW  YORK 
JOHN  WILEY  &  SONS,  INC. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 


TS 


SCfENCfS 
L»RARY 


COPYRIGHT' 1915 • BY 
LOUIS    V.    PIRSSON 

AND 
CHARLES    SCHUCHERT 

COPYRIGHT  -1920 -BY 
ELIZA    BRUSH    PIRSSON 

AND 
CHARLES    SCHUCHERT 


5-24 


PREFACE  TO  PART  I,  SECOND  EDITION 


PREFACE  TO  THE  SECOND  EDITION 

A  new  edition  of  a  textbook,  especially  in  a  scientific  subject  like 
Geology,  in  which  fresh  material  is  constantly  appearing,  demands 
no  particular  explanation.  The  plan  and  scope  of  the  work  remains 
unchanged,  and  in  the  revision  and  the  addition  of  new  matter  in 
many  places  the  effort  has  been  made  to  keep  the  length  of  the 
work  essentially  the  same.* 

The  author  is  indebted  to  various  friends  and  correspondents  for 
corrections,  helpful  criticisms,  and  suggestions  for  betterments,  to 
whom  he  desires  to  extend  his  thanks  and  appreciation  for  the  in- 
terest they  have  shown  and  the  pains  they  have  taken  in  the  mat- 
ter. He  would  like  to  mention  in  this  connection  Mr.  C.  K.  Need- 
ham,  Dr.  H.  H.  Robinson,  who  redrew  several  figures,  and  espe- 
cially Prof.  Douglas  W.  Johnson  of  Columbia  University  for  many 
valued  suggestions. 

In  like  manner  his  thanks  are  due  also  to  his  colleagues,  Professor 
H.  E.  Gregory,  Professor  A.  M.  Bateman  and  the  late  Professor 
Joseph  Barrell. 

Owing  to  illness,  the  revision  of  the  proof  has  been  kindly  under- 
taken by  Professor  Schuchert  and  Miss  Clara  Mae  Le  Vene. 

L.  V.  PIRSSON. 

SHEFFIELD  SCIENTIFIC  SCHOOL  OF  YALE  UNIVERSITY, 

NEW  HAVEN,  CONN., 

May,  1919. 

FROM  THE  PREFACE  TO  THE  FIRST  EDITION 

For  many  years  the  author  of  this  book  has  been  called  upon  to 
give  the  first  course  in  Physical  Geology  to  large  classes  of  students, 
among  whom  are  to  be  found  those  pursuing  courses  leading  to  pro- 
fessional work  in  various  branches  of  Engineering,  Mining,  Metal- 
lurgy, Forestry,  Chemistry,  etc.,  and  in  Geology  itself,  to  whom 
therefore  the  subject  has  a  direct  technical  value  or  serves  as  a  basis 
for  further  technical  studies.  Naturally  these  students  find  a  first 
general  course  in  Physical  Geology  one  of  cultural  interest  as  well. 

In  the  pursuit  of  this  work  the  writer  has  long  felt  the  need  of  a 
*  In  the  present  reprinting  of  this  edition,  Chapter  XVI,  on  Ore  Deposits, 
has  been  rewritten  by  Professor  A.  M.  Bateman. 

v 


vi  PREFACE  TO  PART  I 

textbook  which,  while  presenting  the  broad  facts  and  principles  of 
the  science  from  the  latest  viewpoint,  should  have  a  character 
somewhat  different,  and  a  balance  more  even  in  the  subject  matter 
composing  it,  than  is  to  be  found  in  available  texts. 

Although  original  matter  or  views  of  problems  have  been  incor- 
porated in  places,  it  is  obvious  that  the  preparation  of  a  work  of 
this  nature  must  mainly  be  one  of  selection  of  the  subject  matter 
from  published  material.  It  would  be  impossible  to  give  the  greatly 
varied  sources  from  which  it  has  been  drawn,  but  it  may  be  men- 
tioned that  the  general  treatises  of  Dana,  Geikie,  Chamberlin  and 
Salisbury,  Haug,  Suess,  and  others,  together  with  the  wealth  of 
material  embodied  in  the  reports  and  bulletins  of  the  United  States 
Geological  Survey,  have  been  freely  used,  as  well  as  other  works  in 
special  fields  too  numerous  to  mention. 

For  efficient  help,  freely  given,  in  the  reading  and  preparation  of 
different  parts  of  the  text,  the  author  wishes  to  render  grateful  ac- 
knowledgment to  his  friends  and  colleagues,  Professors  J.  P.  Iddings, 
J.  D.  Irving,  W.  E.  Ford,  and  especially  to  Professor  Joseph  Barrell, 
whose  criticism  and  advice  were  of  the  greatest  service. 

In  the  matter  of  illustrations  the  writer  desires  to  express  his 
obligations  especially  to  Dr.  George  Otis  Smith,  Director  of  the 
United  States  Geological  Survey,  who  placed  at  his  disposal  its  great 
mass  of  photographic  material,  the  proper  credit  for  these  photo- 
graphs being  given  in  each  case;  to  Mr.  J.  J.  H.  Teall,  recent  Di- 
rector of  the  Geological  Survey  of  Great  Britain;  to  Professor  G.  P. 
Merrill  of  Washington,  D.  C.;  to  Professor  J.  E.  Talmage  of  Salt 
Lake  City;  to  Mr.  G.  W.  Grabham  of  Khartoum,  and  to  many  other 
friends  whose  names  are  credited  in  each  case. 

L.  V.  PIRSSON. 

SHEFFIELD  SCIENTIFIC  SCHOOL  OF  YALE  UNIVERSITY, 

NEW  HAVEN,  CONN., 

Dec.,  1914. 


TABLE  OF  CONTENTS 


PART  L  —  PHYSICAL  GEOLOGY 

DIVISION  I.  — DYNAMICAL  GEOLOGY 

CHAPTER  PAGE 

INTRODUCTION 3 

I.   GENERAL  CONSIDERATIONS;   WORK  OF  THE  ATMOSPHERE 8 

II.  RAIN  AND  RUNNING  WATER 31 

III.  LAKES  AND  INTERIOR  DRAINAGE 79 

IV.  THE  OCEAN  AND  ITS  WORK 90 

V.  ICE  AS  A  GEOLOGICAL  AGENCY 118 

VI.  UNDERGROUND  WATER 153 

VII.  THE  GEOLOGICAL  WORK  OP  ORGANIC  LIFE 171 

VIII.  IGNEOUS  AGENCIES;  VOLCANOES  AND  HOT  SPRINGS.  . ; 194 

IX.   MOVEMENTS  OF  THE  EARTH'S  OUTER  SHELL;  EARTHQUAKES 235 

DIVISION  II.  — STRUCTURAL  GEOLOGY 

X.   GENERAL  STRUCTURE  AND  PROPERTIES  OF  THE  EARTH 258 

XI.  SEDIMENTARY  ROCKS 271 

XII.  IGNEOUS  ROCKS * 312 

XIII.  METAMORPHIC  ROCKS 335 

XIV.  FRACTURES  AND  FAULTING  OF  ROCKS 354 

XV.   MOUNTAIN  RANGES;  THEIR  ORIGIN  AND  HISTORY 373 

XVI.  ORE  DEPOSITS 409 

APPENDIX  A 437 

INDEX.  .  453 


Vll 


PART   I 

PHYSICAL  GEOLOGY 

BY 

L.  V.  PIRSSON 


TEXT-BOOK  OF  GEOLOGY 


INTRODUCTION 

GEOLOGY  AS  A  SCIENCE,  AND  ITS  SUB-DIVISIONS 

Geology  is  that  branch  of  Science  which  treats  of  the  Earth, 
comprehensively,  as  a  subject  of  research  and  study.  It  seeks  to 
explain  the  origin  of  the  earth,  especially  in  its  relations  to  other 
planets,  and  to  the  Solar  System  of  which  it  is  a  part;  it  endeavors 
to  account  for  its  varied  surface  features,  for  its  atmosphere,  the 
distribution  of  land  and  water,  its  rivers,  lakes  and  seas,  its  moun- 
tains and  plains.  It  studies  these  features  in  the  light  of  varied 
forces  and  agencies  operating  upon  them,  and  attempts  to  show 
their  history  during  long  ages  past.  It  takes  account  of  the  ma- 
terials composing  the  earth,  and,  from  the  remains  of  plants  and 
animals  still  existing  in  the  rocks,  it  aims  to  present  a  picture  of 
the  successions  of  living  organisms  which  have  existed  during  past 
times  down  to  the  present. 

Geology  is  essentially  an  historical  science  in  that  it  continually 
seeks  to  determine  the  origin  of  things  and  the  changes  which 
they  have  experienced.  The  documents  upon  which  the  history  is 
based  are  written  in  the  rocks  of  the  earth  itself  and  the  forms  of 
its  surface  features;  they  present  a  series  of  incontrovertible 
records,  and,  if  we  would  read  this  history,  it  is  our  part  to  learn 
to  decipher  the  records  correctly.  Much  of  this  has  been  done, 
but  much  also  remains  to  be  done;  it  is  the  aim  of  this  work  to 
present  a  general  account  of  what  has  been  accomplished.  Many 
writers  define  Geology  as  a  history  of  the  earth  and  its  inhabitants, 
as  shown  by  the  record  in  the  rocks. 

Geological  Sciences.  —  From  what  has  been  stated  above  it  is 
clear  that  the  material  treated  in  Geology  is  of  wide  extent,  and 
embraces  a  great  variety  of  subjects.  Hence,  with  the  develop- 
ment of  the  science,  the  increasing  fund  of  information  gained  has 
become  so  extensive  that  different  branches  of  Geology,  or  geo- 
logical sciences,  have  come  to  be  generally  recognized  as  separate 

3 


4  T£X:T-£66K  OF  GEOLOGY 

fields  for  research  and  study.     Some  of  the  more  important  of 
these  are  as  follows: 

Mineralogy,  which  deals  with  the  origin,  composition,  and  prop- 
erties of  inorganic  chemical  compounds,  which  exist  already  formed 
in  the  earth's  crust. 

Petrology,  which  treats  of  the  origin,  properties,  and  relations  of 
the  material  forming  the  various  rock  masses  which  are  com- 
ponent parts  of  the  earth's  crust. 

Meteorology,  the  science  of  the  earth's  atmosphere  and  its  vari- 
ous phenomena,  such  as  variations  of  heat  and  cold,  of  its  moisture, 
and  its  movements,  as  seen  in  winds  and  storms. 

Paleontology,  the  science  which  deals  with  the  life  of  past  ages, 
as  shown  by  the  remains  or  natural  molds  and  imprints  of  plants 
and  animals,  called  fossils,  which  have  been  preserved,  enclosed  in 
the  rocks. 

Physiography,  the  science  which  treats  of  the  present  surface  of 
the  earth  and  seeks  to  understand  the  causes  of  its  relief  features 
and  the  nature  of  the  various  agencies  which  are  at  work  modify- 
ing them.  It  might  indeed  be  called  the  geology  of  the  present. 

Economic  Geology,  which  deals  chiefly  with  the  use  of  the  materials 
of  the  earth's  surface  in  the  service  of  mankind,  and  in  the  appli- 
cation of  geological  facts  and  principles  in  obtaining  them. 

While  the  recognition  of  these  branches  of  science,  which  have' 
developed  from  the  main  stem  of  Geology  as  separate  lines  of  in- 
quiry and  study,  has  narrowed  that  of  Geology  proper,  so  called, 
it  must  yet  be  understood  that  they  are  really  special  phases  of 
the  subject,  intimately  related  to  it,  and  that  some  knowledge  of 
them  is  necessary  for  a  proper  comprehension  of  Geology  in  its 
broader  aspects. 

Sub-divisions  of  Geology.  —  Very  briefly  stated,  Geology  may 
be  considered  thus.  A  mass  of  varied  materials  has  been,  and  is 
being,  acted  upon  by  certain  agencies,  by  which  definite  results 
have  been,  and  are  continuing  to  be,  produced.  We  may  study  and 
determine  the  nature  of  the  materials  operated  upon;  we  may  con- 
sider the  kinds  and  modes  of  operation  of  the  agents  and  forces; 
and,  lastly,  we  may  learn  the  character  and  extent  of  the  results 
achieved.  From  this  it  naturally  follows  that  there  are  three  main 
sub-divisions  of  Geology,  as  follows: 

DYNAMICAL  GEOLOGY,  a  consideration  of  the  facts  and  principles 
concerning  the  various  dynamical  agents,  such  as  wind,  running 
water,  moving  ice,  volcanic  activities,  etc.,  which  operate  upon  the 
earth,  and  modify  its  outer  portion. 


GEOLOGY  AS  A  SCIENCE  5 

STRUCTURAL  GEOLOGY,  an  account  of  the  nature,  properties,  re- 
lations and  positions  of  the  component  rock  masses  of  the  outer 
part  of  the  earth.  It  includes  the  architecture  of  the  outer  shell  of 
the  earth.  These  two,  dynamical  and  structural  geology,  as  opposed 
to  historical  geology,  may  be  classed  together  under  the  general 
heading  of  Physical  Geology. 

HISTORICAL  GEOLOGY,  a  review  of  the  sequence  of  the  events 
which  have  happened  to  the  earth  in  the  past,  as  revealed  by  the 
rocks  and  fossils.  This  includes  Paleogeography,  or  the  varied  dis- 
positions of  land  and  sea  and  their  character  in  former  ages,  and 
Paleontology,  which  has  been  mentioned  above  as  picturing  the 
different  successions  of  organic  life  which  have  inhabited  the  earth. 

It  is  clearly  evident  that  a  knowledge  of  structural  and  dynamical  geology 
requisite  for  a  proper  understanding  of  the  historical  portion  of  the  subject 
and  must  therefore  precede  this.  From  the  purely  philosophic  side  it  would 
seem  natural  to  inquire  into  the  character  of  the  masses  operated  upon  before 
engaging  in  the  study  of  the  forces  modifying  them,  but  it  is  impossible  to  treat 
structural  geology  without  some  reference  to  dynamical  geology;  to  consider  results 
without  considering  causes.  Moreover,  the  various  agencies  which  have  worked 
in  the  past  are  at  work  now,  and  their  operations  are  in  some  degree  familiar 
to  all.  By  thus  treating  dynamical  geology  first  the  mind  is  led  from  the  known 
to  the  unknown,  and  from  the  present  to  the  past.  It  therefore  appears  more 
logical  for  the  beginner  to  study  dynamical  geology  first,  even  though  this  may 
cause  some  repetition  in  succeeding  phases  of  the  subject,  and  this  order  has 
therefore  been  adopted  in  this  work. 

Method  of  Geological  Study.  —  In  former  times  it  was  thought 
that  the  more  prominent  and  striking  features  which  relieve  the 
earth's  surface  were  due  to  some  sudden  and  violent  action.  Thus 
in  surveying  a  deep  canyon,  or  gorge,  scoring  the  earth's  surface, 
or  a  towering  rock  mass  giving  rise  to  a  mountain  peak  or  tall 
cliff,  it  was  customary  to  say  that  this  must  have  been  "caused  by 
some  great  convulsion  of  Nature"  and  this  idea  with  its  phrase  still 
persists,  and  is  frequently  used  by  those  untrained  in  geology.  It 
is  often  seen  in  descriptions  of  natural  scenery.  A  great  convulsion 
of  nature  is  a  cataclysm,  and  it  was  thought  that  the  varied  changes 
which  the  earth's  surface  has  evidently  undergone  were  due  to  a 
series  of  cataclysms,  produced  by  some  sort  of  unknown  and  terri- 
ble forces.  The  error  in  this  method  of  thinking  is  that  the  ele- 
lent  time  is  not  taken  into  account.  A  given  result  may  be  equally 
achieved  by  a  great  force  acting  very  quickly,  or  by  a  small  force 
acting  through  a  long  period  of  time.  It  is  the  triumph  of  Geology, 
as  a  science,  to  have  demonstrated  that  we  do  not  need  to  refer  to 
vast,  unknown,  and  terrible  causes  the  relief  features  of  the  earth, 


6 


TEXT-BOOK  OF  GEOLOGY 


but  that  the  known  agencies  at  work  today  are  competent  to  pro- 
duce them,  provided  they  have  enough  time.  Thus,  we  know,  for  reasons 
we  shall  see  later  on,  that  the  gorge  seen  in  the  accompanying  illus- 


I-  —  Grand  Canyon  of  the  Yellowstone  River. 


tration,  Fig.  1,  has  been  cut  into  the  earth's  surface  by  the  scratch- 
ing of  the  sand  and  gravel  dragged  along  by  the  river  during  the 
lapse  of  a  vast  length  of  time.  Therefore  the  method  of  geological 


GEOLOGY  AS  A  SCIENCE  7 

research  may  be  defined  as  an  inquiry  into  the  past  in  the  light  of  the 
present,  of  the  solving  of  the  unknown  by  the  application  of  the 
known. 

Geologic  Time.  —  The  recognition  of  the  element  of  time  has 
been  stated  above  as  of  fundamental  importance  in  geological  reason- 
ing. Yet  this  generally  involves  a  new  conception  to  one  beginning 
the  study  of  the  science.  As  in  taking  up  the  study  of  Astronomy 
one  has  to  gain  new  ideas  of  distance,  and  to  think  no  longer  on  a 
basis  of  feet,  yards,  and  single  miles,  but  in  terms  of  millions  of 
miles,  so  in  Geology  one  is  compelled  to  think  in  vast  lapses  of  time, 
which  in  many  cases  are  to  be  measured  in  millions  of  years.  Since 
we  have  no  accurate  measures  of  time  in  Geology,  as  we  have  of 
distance  in  Astronomy,  the  phrase  " speaking  geologically"  is  often 
used  with  "great"  or  "small,"  "long"  or  "short"  to  indicate  rela- 
tive lapses  of  what,  from  the  human  standpoint,  may  be  enormous 
periods  of  time.  Thus,  speaking  geologically,  a  million  years  may 
be  relatively  a  short  interval. 

Basal  Sciences.  —  A  subject  so  comprehensive  as  Geology  is 
largely  based  upon  and  has  close  relations  with  other  sciences. 
The  most  fundamental  of  these  are  Chemistry  and  Physics,  some 
knowledge  of  which  is  essential.  Some  acquaintance  with  Min- 
eralogy is  also  highly  desirable,  though  in  a  measure  the  want  of 
this  may  be  supplied  during  the  consideration  of  the  subjects  to 
which  it  applies.  On  the  cosmical  side  Geology  passes  into  Astron- 
omy, and  in  the  study  of  Paleontology  familiarity  with  the  elements 
of  Zoology  and  Botany  is  needed.  Other  subjects,  such  as  Geog- 
raphy and  Mathematics,  might  be  mentioned,  but  it  is  assumed  that 
the  student  has  already  acquired  as  much  of  these  as  is  needed. 


DIVISION  I.  — DYNAMICAL  GEOLOGY 


CHAPTER  I 

GENERAL  CONSIDERATIONS;  THE  ATMOSPHERE  AND 
ITS  WORK 

Dynamical  geology  is  a  consideration  of  the  facts  and  principles 
relating  to  the  different  agencies  which  are  now  modifying  the  surface 
of  the  earth.  They  may  be  broadly  divided  into  two  main  groups; 
external  —  those  whose  controlling  energy  is  derived  from  sources  ex- 
terior to  the  earth,  chiefly  from  the  sun  and  in  lesser  degree  from  the 
moon;  and  internal — those  whose  operations  appear  to  be  due  mainly 
to  the  interior  heat  and  to  the  gravitative  force  of  the  earth.  They 
may  be  classified  as  follows: 

EXTERNAL  AGENCIES  INTERNAL  AGENCIES 

The  Atmosphere.  Underground  Water. 

Rainfall  and  Streams.  Volcanoes. 

Lakes.  Faulting    resulting    in   Earth- 

The  Ocean.  quakes. 

Snow  and  Ice.  Slow  Movements  of  the  Earth's 

Organic  Life.  Crust. 

Rate  of  Work.  —  When  it  came  to  be  appreciated  that  these 
known  agencies  were  sufficient  to  have  produced  the  present  relief 
features  of  the  earth,  and  the  varied  structures  of  its  outer  shell,  as 
investigation  has  revealed  them,  in  a  natural  reaction  from  the 
previous  ideas  that  these  were  due  to  successive  cataclysms,  a  view 
arose  that  these  agencies  had  always  worked  with  great  uniformity, 
at  the  same  rate  and  with  the  same  intensity  that  they  do  today. 
This  view  is  no  longer  held,  as  it  appears  that  in  some  periods  in 
the  past  the  action  of  some  of  them  has  been  more  intensive  than 
in  other  periods,  and  it  seems  probable  that,  while  on  the  whole  the 
energy  has  been  declining,  that  of  some  has  been  gradually  in- 
creasing. The  reasons  for  thinking  this  will  appear  in  the  course  of 
this  work,  as  the  different  subjects  are  taken  up. 

The  actual  rate  at  which  geological  work  is  accomplished,  from 
the  human  standpoint,  is,  in  general,  very  slow.  Of  course  in  some 
cases,  as  where  in  a  volcanic  eruption,  a  very  large  amount  of  matter 

8 


THE  ATMOSPHERE  AND  ITS  WORK  9 

is  suddenly  transferred  from  the  inside  to  the  outside  of  the  earth, 
the  work  done  is  not  only  evident,  but  startling.  The  same  would 
be  true  for  instance  in  the  case  of  heavy  landslides.  But,  in  general, 
the  amount  of  work  done  at  this  rate  is  small,  compared  with  that 
accomplished,  much  of  it  imperceptibly,  most  of  it  so  slowly,  that  it 
is  only  in  viewing  the  results  achieved  that  we  can  truly  judge  of 
its  extent.  As  in  looking  at  the  hour  hand  of  a  clock  we  see  no 
perceptible  movement  at  a  given  instant  and  yet  know  by  compari- 
son the  movement  is  taking  place,  so  we  infer  that  many  geological 
processes  have  been  very  slowly,  but  none  the  less  steadily  and 
ceaselessly  occurring.  It  is  the  recognition  of  this  that  forces  us  to 
acknowledge  the  lapse  of  immensely  long  periods  of  time,  as  pre- 
viously stated  in  the  introduction. 

THE  ATMOSPHERE  AND  ITS  WORK 

Character  and  Composition.  —  The  atmosphere  is  the  outer 
gaseous  envelope  of  the  earth.  Owing  to  the  compressibility  of 
gases  it  is  densest  at  sea-level,  where  it  exerts  an  average  pressure 
of  nearly  15  pounds  to  the  square  inch.  It  regularly  decreases  in 
density  as  one  ascends,  but  its  height  is  not  known;  however,  since 
meteors  passing  through  space,  on  coming  in  contact  with  it,  be- 
come heated  and  luminous,  and  exhibit  this  phenomenon  at  least 
100  miles  above  the  earth,  it  certainly  extends  upward  to  this  point 
in  appreciable  quantity  and  in  more  diffuse  form  to  considerably 


Fig.   2.  —  Diagram  showing  a  segment  of  the  earth  with  the  atmosphere  50 
miles  high  in  true  proportion. 

greater  heights.  At  50  miles  it  is  extremely  rare,  and  at  about  3.6 
miles  (19,000  feet)  its  density  is  only  one  half  that  at  sea-level,  that 
is  to  say  one  half  of  the  actual  amount  of  air  lies  below  this  level. 

In  composition  the  air  consists  of  about  four  parts  of  nitrogen  to 
one  of  oxygen.  Although  these  are  the  chief  elements  there  are  also 
carbonic  acid  gas,  CC>2,  in  the  proportion  of  about  3  volumes  in 
10,000  of  air,  and  water  vapor  whose  quantity  varies  according  to 
temperature,  locality,  and  season;  under  ordinary  conditions  in  our 
living  rooms  a  cubic  yard  of  air  carries  from  1/5  to  2/5  of  an  ounce 
of  water,  or  from  about  one  to  two  tablespoonfuls.  In  addition 
there  are  relatively  minute  amounts  of  other  gases  and  volatile 


10  TEXT-BOOK  OF  GEOLOGY 

compounds  in  the  air,  but  these  are  not  of  geological  importance, 
and  may  be  disregarded  in  this  connection. 

Origin  of  the  Atmosphere.  —  A  discussion  of  the  origin  of  the  atmosphere 
must  in  some  measure  involve  that  of  the  earth  itself,  and  while  it  is  inadvisable 
to  consider  the  latter  until  later,  when  the  student  is  better  prepared  for  it,  the 
following  considerations  are  of  interest  in  this  connection.  Of  the  different 
views  which  have  been  held  regarding  the  atmosphere's  origin  no  one  has,  as 
yet,  received  recognition  as  fully  satisfactory.  They  may  be  roughly  classed 
into  two  groups.  According  to  the  first,  the  origin  of  the  atmosphere  dates  back 
to  that  of  the  earth.  It  is  held  that  the  matter  composing  the  earth  was  once  a 
great  mass  of  extended,  heated  gas  and  vapor,  part  of  a  larger  mass  which  formed 
the  solar  system.  As  this  cooled  and  contracted  it  eventually  produced  our 
solid  earth,  but  the  part  which  still  remained  gaseous  now  forms  our  atmosphere. 
Thus  the  latter  is  thought  to  be  coeval  with  the  earth. 

According  to  the  other  view  the  earth  had  originally  little  or  no  atmosphere; 
the  gases  which  compose  it  being  held  occluded,  that  is  absorbed,  in  its  mass, 
and  as  the  earth  has  contracted,  either  through  cooling  and  crystallizing  or 
through  gravitative  force,  they  have  been  excluded,  squeezed  out,  and  now  form 
the  atmosphere.  This  view,  in  part,  might  be  illustrated  by  the  action  of  silver 
which,  when  melted,  absorbs  oxygen  from  the  air  and  holds  it  occluded;  when 
it  solidifies  the  gas  is  again  returned  to  the  air  with  some  violence. 

Following  the  first  view  the  atmosphere,  especially  its  content  in  carbon 
dioxide,  has  been  gradually  diminishing  in  amount;  following  the  second  it  has 
been  gradually  supplied.  Various  modifications  of  these  views,  especially  en- 
deavoring to  account  for  variations  in  the  amount  of  oxygen,  water  vapor,  and 
carbon  dioxide,  which  are  the  substances  chiefly  important  as  agents  in  geological 
processes,  have  been  suggested.  Thus  for  example  it  has  been  held,  since  vege- 
table life  takes  carbon  dioxide  from  the  atmosphere  and  decomposes  it,  storing 
up  carbon  and  liberating  oxygen,  that  originally  the  atmosphere  was  full  of 
carbon  dioxide  and  deficient,  or  wanting,  in  oxygen,  but  that  through  this  action 
of  plants  the  conditions  have  been  gradually  reversed.  The  student  should, 
however,  remember  that  these  views  are  hypothetical,  and  that  science  is  not  yet 
able  to  pronounce  authoritatively  upon  them.  Where  they  concern  the  aspect 
of  special  questions  they  will  be  considered  in  detail  in  their  appropriate  places. 

Importance  of  the  Atmosphere.  —  The  atmosphere  is  an  agent 
of  the  highest  importance  in  surface  geological  processes.  Not  only 
does  it  work  directly  in  both  a  destructive  and  a  constructive  way, 
but  without  it  there  could  be  no  work  from  rainfall  and  running 
water,  as  we  now  know  it,  and  the  activities  of  plant  and  animal 
life  would  cease.  Water,  without  at  least  an  atmosphere  of  water 
vapor  above  it,  could  not  remain  on  the  surface  of  the  globe,  which 
would  then  be  dead  and  inert,  and  surface  changes,  due  to  external 
agencies,  would  not  occur.  This  is  illustrated  by  the  moon,  which 
appears  to  have  no  atmosphere  or  water  upon  it,  due  apparently 
to  the  fact  that  its  mass  is  not  great  enough  to  exert  sufficient 
force  of  gravity  to  retain  around  it  the  gases  which  might  have 
formed  its  atmosphere.  Its  surface  features,  as  revealed  by  the 


THE    ATMOSPHERE    AND    ITS    WORK  H 

most  powerful  telescopes,  seem  to  be  those  produced  by  internal 
agencies,  largely  volcanic  in  nature,  and  by  the  impact  of  meteoric 
bodies  from  space,  unmodified  by  later  changes  due  to  atmospheric 
effects. 

Movements  in  the  Atmosphere. —  The  unequal  heating  of  the 
atmosphere  in  tropical  and  polar  regions  gives  rise  to  movements 
in  it,  which  cause  circulation  on  a  large  scale.  Heated  in  the  tropics 
the  air  expands,  rises,  and  flows  off  toward  the  poles;  it  then  cools, 
contracts,  and  becoming  denser  sinks,  and  moves  back  to  the 
tropics.  The  principle  is  the  familiar  one  of  convection  currents  in 
fluids.  The  circulation  thus  established,  poleward  above  and  equa- 
torward  below,  is,  however,  greatly  modified  by  the  rotation  of  the 
earth,  which  deflects  the  north  and  south  movements,  or  air  cur- 
rents, eastward  and  westward,  giving  rise  to  belts  of  prevalent 
winds,  parallel  to  the  equator. 

North  and  south  of  the  equator  are  belts,  extending  to  28°  of  latitude,  of 
so-called  trade  winds,  which  blow  in  the  northern  belt  from  the  northeast, 
in  the  southern  belt  from  the  southeast.  They  are  steady,  dry  winds  of  from 
10  to  30  miles  an  hour,  not  often  changed  by  storms.  Where  they  meet 
along  the  equator  there  is  a  narrow  belt  of  calms,  with  occasional  light 
breezes.  In  passing  from  colder  to  warmer  regions  the  air  expands  and  its 
capacity  to  absorb  moisture  increases.  Due  to  this  warm  dry  nature  of  the 
trade  winds,  therefore,  the  lower  lands  of  the  continents  lying  in  their  belts 
have  an  arid  climate  and  desert  character,  like  Sahara  and  western  Australia, 
but  mountainous  regions,  especially  on  the  eastern  side,  like  the  central 
Andes,  are  well  watered. 

North  and  south  of  the  belts  of  trade  winds  in  each  hemisphere  is  one  cov- 
ering the  temperate  regions  in  which  the  winds  are  prevailing  westerly. 
They  are  of  variable  strength,  from  10  up  to  60  miles  an  hour,  and  their 
regularity  is  much  interfered  with  by  great  whirls,  or  eddies,  known  as  cy- 
clones, moving  in  a  general  easterly  direction,  which  give  rise  to  storms. 
These  westerlies  and  their  storms  determine  the  climate  of  most  of  the  United 
States  and  southern  Canada. 

The  orderly  courses  of  the  atmospheric  circulation  described  are  however 
considerably  modified  by  the  arrangement  of  the  continents  and  oceans,  and 
by  the  change  of  seasons  from  summer  to  winter.  The  latter  causes  a  shift- 
ing of  the  wind  belts  northward  or  southward,  while  the  unequal  heating  of 
the  air  over  land  and  sea  areas  by  the  sun  also  has  its  effects,  as  in  local 
winds  such  as  land  and  sea  breezes. 

Air  movements  about  the  poles  are  less  well  known,  and  of  lesser  impor- 
tance, from  economic  and  geologic  standpoints. 

Work  of  the  Atmosphere.  —  It  has  been  stated  above  that  the 
work  of  the  atmosphere  may  be  regarded  as  both  destructive  and 
constructive.  The  former  consists  in  its  chemical  action  upon  rocks 
and  minerals,  whereby  former  chemical  compounds  are  broken  up 


12 


TEXT-BOOK   OF   GEOLOGY 


and  new  ones  formed  in  their  places,  and  in  its  mechanical  activity 
by  which  material  driven  by  the  wind  is  not  only  transported  but 
abrades  and  wears  away  exposed  rock  surfaces.  Its  chemical  action 
is  so  greatly  aided  by  water  coming  in  the  form  of  rain  and  by  the 
expansive  power  of  frost  that  it  is  difficult  to  separate  these  agencies 
and  they  will  consequently  be  considered  later  under  the  general 
term  of  weathering.  The  constructive  work  is  performed  by  the 
wind,  which  is  a  factor  of  considerable  importance  in  transporting 
and  depositing  material.  This  classification  may  be  shown  in  a 
table  as  follows: 

WORK  OF  THE  ATMOSPHERE 

f  Chemical,  Decay  of  Rocks  (Weathering). 
Destructive  < 

I  Mechanical,  Wearing  of  Rocks  (Wind-driven  Sand  and  Waves). 

Constructive,  Transport  and  Deposit,  Formation  of  Dunes,  Loess,  etc. 


Destructive    Work.  —  Omitting   for   the   present   the   work   of 
weathering  and  of  the  waves,  which  are  better  considered  in  con- 


Fig.  3.  —  Looking  Glass  Rock,  near  La  Sal  Mts.,  Utah.  White  horse  near  tree 
gives  scale.  Cut  and  worn  to  its  present  shape,  in  part  by  the  action  of  the  wind. 
W.  Cross,  U.  S.  Geol.  Surv. 

nection  with  that  of  water,  the  direct  destructive  effects  of  the  atmo- 
sphere as  a  geological  agent  are  best  seen  in  those  places  where  sand 


THE    ATMOSPHERE    AND    ITS    WORK  13 

driven  by  the  wind  wears  away  exposed  rock  surfaces.  In  humid 
regions,  where  the  rainfall  promotes  the  growth  of  abundant  vege- 
tation, the  soil  is  protected,  the  wind  is  unable  to  lift  and  carry  it, 
and  thus  having  no  tool  to  work  with,  its  abrasive  effects  are  neg- 
ligible, or  wanting.  Moreover,  in  such  regions  exposed  rock  sur- 
faces are  less  conspicuous,  and  are  apt  to  be  covered  by  a  mat  of 
plant  life  which  serves  as  a  cushion  to  protect  them.  In  arid  regions 
on  the  contrary,  where  there  is  little  or  no  rainfall,  vegetation  is 
scanty  or  lacking  and  the  loose  soil  is  constantly  being  shifted  by 
the  wind  and  driven  against  the  exposed  rock-masses.  This  is,  of 
course,  most  strikingly  seen  in  deserts.  In  such  places  rocks  or 
bowlders  outcropping  from  the  soil  are  worn  and  polished  by  the 
sand  drifting  past  and  over  them.  In  arid  countries,  as  in  the  south- 
west part  of  the  United  States,  the  walls  of  rock  masses  are  carved 
and  cut  into  hollows  and  caves  by  the  disintegration  of  the  rock  by 
chemical  and  physical  processes  described  later,  and  the  removal 
of  the  loosened  material  by  the  wind.  See  Fig.  3.  The  pounding  of 
sand  grains  also  aids  in  wearing  away  the  softer  parts  of  the  rock 
leaving  the  harder  ones  projecting,  often  in  intricate  fret- work. 
Thus  the  wind  helps  in  the  general  process  of  rock  decay  by  carry- 
ing material  away,  and  exposing  fresh  surfaces  to  the  attack  of 
both  wind  and  weather.  The  efficiency  of  the  wind  as  a  factor  in 
the  wearing  away  of  land  surfaces  and  transporting  material  in 
arid  regions  has  probably  been  undervalued. 

The  rate  at  which  cutting  is  carried  on  under  favorable  conditions 
may  be  faster  than  would  at  first  be  imagined.  We  may  gain  some 
idea  of  it  from  the  fact  that  the  window  glass  of  houses  along  sea- 
shores, which  are  directly  exposed  to  storm-driven  sand,  may  lose 
their  transparency  in  a  day  or  two  and  be  completely  penetrated  in  a 
month  or  so.  It  was  the  observation  of  this  that  led  to  the  use  of 
the  artificial  sand-blast  as  an  instrument  for  the  etching  of  glass 
and  stone.  Telegraph  poles  planted  in  the  desert  are  quickly  cut 
down  by  the  sand  drifting  past  their  bases.  The  chief  destructive 
work  of  the  wind,  however,  is  not  so  much  in  actually  abrading 
surfaces,  as  in  removing  material  loosened  by  other  means. 

Constructive  Work.  —  This  is  illustrated  in  the  deposits  formed 
by  the  wind  from  transported  material.  A  gentle  breeze  lifts  and 
carries  dust,  a  strong  wind  drives  sand  along  with  it,  while  a  tem- 
pest may  move  gravel  the  size  of  peas.  There  may  be  transported 
in  this  way  vast  quantities  of  material.  See  Fig.  4.  For  instance, 
it  has  been  shown  that  a  single  storm,  travelling  from  the  arid 
southwest  a  thousand  miles  into  the  region  about  the  Great  Lakes, 


14  TEXT-BOOK    OF    GEOLOGY 

brought  with  it  a  million  tons  of  dust,  and  probably  a  much  larger 
quantity,  which  was  deposited  in  the  snowfall  over  a  wide  area. 
Such  material,  dropped  as  the  wind  slackens,  under  favorable  con- 
ditions., may  form  deposits  of  great  magnitude.  They  are  known 


Fig.  4.  —  Sand-storm  sweeping  over  Khartoum  North:  in  front  is  the  Blue  Nile. 
Shows  enormous  transporting  power  of  the  wind.  Soudan,  June  6,  1906.  (Photo 
by  Wm.  Beam,  M.D.) 

as  eolian  (^Eolus,  god  of  the  winds)  deposits,  a  term  used  to  dis- 
tinguish them  from  sedimentary  deposits  formed  in  water.  They 
are  most  prominently  illustrated  in  dunes  and  in  the  loess. 

Dunes.  —  Sand-hills,  or  dunes,  are  hillocks,  or  hills,  made  by 
wind-borne  sand  in  a  manner  similar  to  that  in  which  snow  forms 
drifts.  They  vary  in  height  from  a  few  feet  up  to  100,  or  even  200, 
feet  or  more.  The  sand  grains  composing  them  are  mainly  of 
quartz,*  though  a  variety  of  other  minerals  may  occur,  rounded  by 
the  rolling  and  abrading  action  they  have  undergone.  The  starting 
of  a  dune  may  have  been  caused  by  some  obstacle,  such  as  a  stump 
or  stone,  causing  a  temporary  lull  in  the  wind  behind  it.  Sand  is 
here  deposited  and  the  dune,  once  begun,  continues  to  grow.  In 
regions  where  they  occur  the  erection  of  buildings  has  in  this  way 
started  their  formation.  The  surface  of  a  dune  is  very  apt  to  be 
covered  with  fine  parallel  ridges  of  sand  an  inch  or  so  in  height, 
transverse  to  the  direction  of  the  prevailing  wind,  and  called  ripple- 

*  If  the  student  is  not  acquainted  with  the  ordinary  rock-minerals  he  may 
gain  such  acquaintance  with  them  as  is  necessary  in  the  study  of  this  work  by 
referring  to  Appendix  A. 


THE    ATMOSPHERE    AND    ITS    WORK 


15 


marks,  because  they  are  similar  to  the  fine  parallel  ridges  made  on 
sandy  bottoms  by  the  action  of  waves.  See  Fig.  5.  Dunes  are  found 
along  low  coast  lines  in  all  parts  of  the  world,  where  the  sand  made  by 
the  waves  is  washed  ashore,  and,  caught  up  by  the  prevailing  winds 
from  the  ocean,  is  drifted  inland  and  accumulated.  Thus  they 
occur  at  various  places  along  the  Atlantic  coast,  and  on  the  Pacific 


Fig.  5.  —  View  of  sand-dunes,  near  Mammoth  Station,  Cal.,  showing  ripple-marks. 
W.  C.  Mendenhall,  U.  S.  Geol.  Surv. 

shore  of  the  United  States;  in  England;  on  the  shores  of  the  Baltic 
Sea ;  in  Holland,  France,  etc.  In  the  same  way  they  are  produced 
on  the  shore-lines  of  large  lakes  or  inland  seas;  thus  the  southern 
end  of  Lake  Michigan  is  fringed  with  high  sand-dunes. 

In  arid  regions  where  the  soil,  formed  by  the  disintegration  of  the 
underlying  rocks,  is  not  held  down  by  a  sufficiently  protective 
mantle  of  vegetation  and  is  therefore  easily  moved  by  the  wind  and 
accumulated  in  favorable  places,  sand-dunes  are  a  common  phe- 
nomenon. They  are  thus  characteristic  features  of  desert  land- 
scapes, and  the  great  deserts  of  central  Asia,  of  Africa,  of  Australia, 
and  those  in  western  America  are  in  considerable  part  covered  with 
them.  The  areas  from  which  the  soil  is  moved  are  left  as  barren 
stony  wastes. 

Shape  of  Dunes.  —  The  shape  of  the  dune  varies  according  to 
local  circumstances  and  is  commonly  irregular;  one  form  called 
a  barchane  is  seen  outlined  in  the  ground  plan  in  Fig.  6.  The  arrow 


16 


TEXT-BOOK    OF    GEOLOGY 


shows  the  direction  of  the  prevailing  winds.  The  windward  side  a 
has  a  gentle  slope  whose  angle  depends  on  the 
average  strength  of  the  wind;  if  it  is  very  strong 
the  sand  will  be  carried  up  a  steeper  angle  of 
slope.  The  dune  is  terminated  by  a  rather  sharp 
crest.  On  the  leeward  side  b  is  a  relative  calm 
with  a  back  eddy  and  the  sand  is  here  dropped; 
the  angle  of  slope  is  here  much  steeper,  being  that 
at  which  sand  will  lie  at  rest  without  sliding 
down,  from  20°  to  25°,*  the  down-sliding  being, 
in  part,  arrested  by  the  eddy.  From  this  ideal 
condition  the  shape  is  being  constantly  modified  more  or  less  by 
shifting  winds. 

Migration  of  Dunes.  —  The  transference  of  material  from  the 
windward  to  the  leeward  side  causes  dunes  to  march  steadily  in  the 
direction  of  the  prevailing  wind,  unless  the  sand  is  held  down  by  a 
mat  of  vegetation.  As  the  sand  is  lifted  by  the  wind  and  then 


Fig.  6.  —  Shapes  of 
sand-dunes,  called 
barchanes. 


Church  of  Kunzen 


In  1809 


Place  of  buried  church 


Ruins  of  Church 


Fig.  7.  —  Movement  of  a  sand-dune  during  60  years  on  the  east  shore  of  the 
Baltic  Sea  at  the  village  of  Kunzen.     (After  Berendt.) 

dropped,  the  dunes  maintain  their  height,  or,  with  increase  of  ma- 
terial, grow  higher.  Along  exposed  coasts  the  prevailing  winds  are 
from  the  sea  and  thus  the  shore-line,  especially  where  low  and 
sandy,  is  apt  to  have  a  fringing  belt  of  sand-dunes  which  may  vary 
from  a  few  hundred  yards,  or  less,  to  a  number  of  miles  in  width. 
They  tend  constantly  to  move  inland,  the  rate  of  movement  depend- 
ing on  the  force  of  the  wind;  in  Denmark  they  have  been  found  to 
move  as  much  as  24  feet  in  a  year,  in  other  places  15  feet  or  less. 

*  The  steepest  angle  of  repose  that  could  be  obtained  by  carefully  pouring 
dry  dune  sand  from  San  Francisco  was  about  30°,  in  dunes  this  is  probably 
not  often  obtained. 


THE    ATMOSPHERE    AND    ITS    WORK 


17 


In  their  march  they  cover  and  destroy  arable  lands,  forests,  and 
even  villages  and  towns,  leaving  ruined  sandy  wastes  behind  them. 
Many  instances  of  this  could  be  cited  from  various  parts  of  the 
world;  some  of  the  best  known  are  from  the  shores  of  the  Baltic 
Sea,  See  Fig.  7.  In  the  deserts  of  central  Asia  Sven  Hedin,  the 
explorer,  found  ruined  cities  of  an  ancient  civilization  emerging  from 
the  sand,  which  a  long  period  ago  had  overwhelmed  them  and  the 
fertile  lands  which  must  have  once  supported  them. 


Fig.  8.  —  Forest  overwhelmed,  covered,  killed,  and  then  left  exposed  by  marching 
sand-dunes.     Manitou  Island,  Lake  Michigan.     I.  C.  Russell,  U.  S.  Geol.  Surv. 

When  covered  with  vegetation  the  dunes  are  stationary,  or  move 
more  slowly,  and  therefore  when  they  become  a  menace  attempts 
are  usually  made,  and  often  successfully,  to  induce  such  a  growth 
upon  them.  This  has  been  done  in  places  on  the  Pacific  coast,  and  it 
is  stated  that  the  effect  of  starting  forest  growth  on  the  dunes  along 
the  coast  of  France  to  render  them  stationary  resulted,  not  only  in 
accomplishing  this  purpose,  but  so  profitably  in  regard  to  the  forest 
itself,  as  to  greatly  help  in  inducing  reforestation  elsewhere. 

Loess.  —  In  the  valley  of  the  Rhine  and  other  rivers  of  northern 
Europe  there  occur  in  places  considerable  deposits,  on  the  valley 
sides,  and  even  up  to  great  heights  on  the  slopes  of  mountains,  of  a 
peculiar  structureless,  yellowish-brown  earth  to  which  the  name  of 
loess  has  been  given.  The  particles  of  quartz,  feldspar,  clay,  cal- 


18  TEXT-BOOK    OF    GEOLOGY 

cite,  mica,  and  other  minerals  *  composing  it  are  much  finer  than 
those  of  ordinary  sand  and  are  sharply  angular,  showing  no  sign 
of  rounding  by  wear  as  the  larger  grains  carried  by  wind  and 
water  do.  Nor  do  the  deposits  exhibit  the  lines  of  stratification,  or 
bedding,  which  are  characteristic,  as  will  be  shown  later,  of  the 
sediments  laid  down  by  water.  Moreover  the  shells  found  in  it  are 
those  of  land  forms,  like  snails,  and  the  bones  those  of  land  animals. 
These  facts,  and  its  irregular  distribution  at  various  heights,  appear 
to  prove  that  it  is  a  deposit  which  was  formed  on  land,  not  in  water. 

The  loess  is  full  of  small,  slender,  perpendicular  holes,  or  tubes, 
which  give  it  a  vertical  cleavage,  so  that  it  commonly  presents  in 
many  places  upright  bluffs  along  ravines  and  river  courses,  which, 
depending  on  the  thickness  of  the  deposit,  may  be  of  considerable 
height. 

Similar  deposits  are  found  in  the  United  States  in  the  central 
part  of  the  Mississippi  valley,  especially  in  the  states  of  Iowa, 
Kansas  and  Nebraska,  and  covering  in  sum  total  thousands  of 
square  miles.  They  also  occur  in  Mie  states  of  Oregon  and  Wash- 
ington and  other  parts  of  the  western  United  States.  The  thickness 
is  usually  not  great;  from  10  to  20  feet  perhaps,  sometimes  as  much 
as  100. 

It  is  now  generally  believed  that  the  loess  of  Europe  and  America 
is  for  the  most  part  an  eolian  deposit,  dust  blown  and  dropped  in 
favoring  localities  by  the  wind,  and  accumulated  during  long  periods 
of  time.  The  origin  of  the  material  is  supposed  to  be  as  follows: 
It  is  known  that  in  a  recent  period,  as  will  be  shown  later,  large 
areas  of  North  America  and  Europe  were  covered  with  thick  and 
moving  sheets  of  ice  which  ground  up  the  underlying  rock  and  soil. 
The  fine  material  thus  produced  was  carried  outward  and  beyond  by 
waters  resulting  from  the  melting  of  the  ice,  and  when  spread  out 
in  the  open  valleys  and  land  stretches  it  was,  when  dry,  whirled 
away  in  dust  clouds  by  the  wind  and  deposited.  The  stems  and 
roots  of  successive  generations  of  grasses  growing  on  the  deposits 
and  buried  by  the  rising  accumulations  have  by  their  decay  pro- 
duced the  slender  vertical  tubes  which  have  been  mentioned  above 
as  occurring  in  the  loess. 

The  greatest  development  of  the  loess  is  in  Asia,  in  Turkestan,  Mongolia, 
and  especially  China.  The  greater  part  of  northern  central  China  is  covered 
with  it,  and  the  yellow  earth  washed  down  by  the  rain  and  streams  colors 
the  waters  of  the  great  river  Hoangho  (Yellow  River),  and  the  sea  (Yellow 

*  See  Appendix  A  for  description  of  these  minerals. 


THE    ATMOSPHERE    AND    ITS    WORK 


19 


Sea)  into  which  it  discharges,  and  has  thus  occasioned  their  names.  The 
bluffs,  which  it  forms,  are  in  places  500  feet  high,  and  its  thickness  is  esti- 
mated to  be  greater  than  this  in  some  parts.  In  the  river  valleys  it  commonly 
forms  a  series  of  terraces,  rising  step-like  above  one  another,  with  upright 
bluffs  facing  the  river.  The  Chinese,  who  cultivate  the  arable  soil  it  forms, 
have  cut  back  into  these  bluffs  and  fashioned  cavelike  dwellings  for  them- 
selves, which  have  been  inhabited  for  centuries,  as  seen  in  Fig  9.  Owing  to  the 
vertical  cleavage  and  softness  of  the  loess  the  streams,  even  small  ones,  run  in 
steep-walled  gorges,  while  the  roads  and  paths  which  have  been  used  for  cen- 
turies, by  the  rapid  wear  of  the  soft  material  and  its  constant  removal,  when 


Fig. 


).  —  Dwellings  in  the  Loess  in  Shansi,  China. 
U.  S.  Nat.  Mus. 


Photo  by  Bailey  Willis, 


thus  loosened  by  wind  and  rain  wash,  have  also  become  small  canyons.  The 
whole  country  is  thus  dissected  by  innumerable  ravines  and  gorges,  which 
render  it  impassable  to  the  traveler,  unless  accompanied  by  a  guide. 

The  loess  of  China  was  held  by  von  Richthofen,  the  German  geologist  and 
explorer,  to  have  been  produced  by  dust,  continually  borne  from  the  great 
deserts  of  central  Asia  during  long  ages  by  the  prevailing  winds,  and  deposited 
in  the  basins  and  valleys  where  it  now  lies.  Some  hold,  however,  that  the 
loess,  both  here  and  elsewhere,  is  in  large  measure,  if  not  entirely,  a  deposit 
made  by  water. 


Other  geological  effects  of  the  atmosphere  in  accumulating  de- 
posits of  volcanic  dust,  of  tornadoes  levelling  forests  and  thereby 
impeding  drainage,  etc.,  might  be  mentioned,  but  these  are  of  less 
importance.  The  phases  described  illustrate  sufficiently  its  mechan- 
ical work  and  we  are  now  ready  to  consider  its  chemical  action, 


20  TEXT-BOOK   OF   GEOLOGY 

especially  when  aided  by  water  acting  both  chemically  and  mechan- 
ically.   This  is  seen  in  the  phenomenon  called  weathering. 

ROCK  WEATHERING  AND  SOIL  FORMATION 

The  Soil  Mantle.  —  The  outer  shell  of  the  earth,  as  we  know  it, 
is  everywhere  composed  of  more  or  less  firm  solid  rock,  commonly 
called  for  any  particular  place  "country  rock"  or  "bed-rock." 
Nearly  everywhere  this  is  covered  by  a  mantle  of  loose  material  of 
variable  thickness  which  passes  under  a  variety  of  names,  but  which 
for  convenience,  where  exposed  to  the  air,  we  may  designate  as 
"soil."  Here  and  there  in  ledges,  precipices,  and  the  craggy  tops  of 
hills  and  mountains,  we  may  see  the  bed-rock  projecting  above  this 
mantle  of  soil.  As  compared  with  the  earth,  as  a  whole,  it  is  a  mere 
film  on  its  outer  surface  and  might  be  compared  to  the  film  of  tar- 
nish a  polished  metal  ball  would  acquire  on  exposure  to  moist  air. 
The  part  which  it  plays  in  geological  processes  will  be  considered 
later;  it  is  our  purpose  now  to  study  its  origin,  for  in  its  formation 
is  seen  one  of  the  most  important  functions  of  the  atmosphere  as  a 
geological  agent. 

Weathering.  —  The  outer  rocky  crust,  or  bed-rock,  is  every- 
where more  or  less  shattered;  it  is  penetrated  in  all  directions  by 
cracks  and  fissures,  some  great,  some  small.  Even  the  mineral 
grains  are  more  or  less  filled  with  cracks,  often  cleavage  cracks. 
Into  such  fissures  the  air  enters,  carrying  with  it  the  various  gases 
and  the  insensible  moisture  it  contains.  If  the  moisture  becomes 
sensible,  as  in  the  form  of  rain,  then  water  enters  them  and  by  the 
force  of  capillary  attraction  may  be  drawn  into  the  most  minute 
crevices.  Since  the  air  contains  water,  and  water,  as  a  liquid,  has 
the  power  of  dissolving  gases,  and  therefore  contains  those  of  the  air, 
the  work  of  these  agencies,  air  and  water,  is  so  closely  associated 
that  we  cannot  draw  any  sharp  line  between  them.  They  work  to- 
gether to  cause  rock  to  decay  and  turn  into  soil,  and  in  this  they 
are  powerfully  aided  by  changes  of  temperature,  by  heat  and  by 
cold,  by  substances  carried  in  solution,  and  to  a  lesser  degree  by  the 
action  of  plants  and  animals.  The  work  is  partly  mechanical, 
partly  chemical,  and  taken  altogether  it  comprises  a  rather  complex 
set  of  processes  which  are  conveniently  designated  under  the  name 
of  weathering.  Some  of  these  may  be  considered  separately. 

Heat  and  Cold.  —  The  daily  range  of  temperature,  the  difference 
between  the  heat  of  day  and  the  cold  of  night,  may  be  50°  or  even 
as  much  as  75° ;  the  annual  range,  between  the  cold  of  winter  and 
the  heat  of  summer,  may  be  100°,  or  even  as  much  as  150°.  Where 


THE    ATMOSPHERE    AND    ITS    WORK 


21 


the  rock  masses  are  exposed  to  such  changes  of  temperature  they  are 
powerfully,  irresistibly  expanded  and  contracted,  see  Fig.  10.  A 
mass  of  granite  100  feet  long  by  a  change  of  150°  would  expand  one 
inch.  Moreover  the  unlike  mineral  grains  composing  most  rocks  do 
not  expand  equally,  and  hence  interior  stresses  are,  produced.  Un- 
able to  withstand  such  actions  the  rocks  are  ruptured  and  break 
into  smaller  pieces.  Such  effects  are  not  felt  deeply,  for  rocks  are 
poor  conductors  of  heat,  and  thus  when  bed-rock  is  exposed  the 

r    .  • 


^L  % 

ff 


Fig.  10.  —  Buckling  in  sandstone  layers  due  to  expansion  from  heating  by  the  sun. 
Wyoming.     E.  E.  Smith,  U.  S.  Geol.  Surv. 

back  and  forth  expansion  movements  of  the  surface  layer  tend  to 
shear  it  loose  from  the  unchanging  mass  below.  Thus  the  surface 
crumbles,  or  layers  scale  off,  or  exfoliate,  as  seen  in  Fig.  11.  By 
this  process,  in  those  places  where  great  extremes  of  temperature 
occur,  as  in  deserts  and  in  semi-arid  regions,  the  exposed  rock 
masses  are  broken,  rounded  off,  and  disintegrate  into  soil  and  gravel. 

This  process  breaks  up  the  rock  mechanically  without  chemically  changing 
the  constituent  mineral  grains  and  is  known  as  disintegration.  It  may  be 
aided  to  some  extent  by  the  deposition  of  salts  in  the  pores  and  cracks  in 
the  rocks,  since  in  arid  regions  the  former  tend  to  be  drawn  to  the  surface 
and  left  by  evaporation.  Then  comes  the  wind  which  carries  the  finer  ma- 
terial away,  as  explained  in  a  foregoing  section. 

Effect  of  Frost.  —  In  cold  countries  the  effects  of  disintegration 
described  above  are  greatly  aided  by  the  action  of  frost.  Water 
fills  the  crevices  and  on  freezing  expands,  splitting  the  rocks  with 
great  force.  This  action  is  best  seen  in  high  mountains  whose  slopes 


22 


THE  ATMOSPHERE  AND  ITS  WORK 


Fig.  11.  —  Exfoliation,  or  scaling  of  rock,  by  alternate  expansion  and  contraction  of 
surface  layers.     Nevada  City,  Cal.    G,  K.  Gilbert,  U.  S.  Geol.  Surv, 


THE    ATMOSPHERE    AND    ITS    WORK 


23 


are  in  different  places,  and  sometimes  entirely,  covered  with  such 
broken  rock  fragments,  commonly  called  "slide-rock."  Where  such 
masses  of  debris  accumulate  at  the  foot  of  a  cliff  they  are  called 
talus,  as  seen  in  Fig.  12.  In  high  mountain  ranges  the  effect  of 
frost  in  carving  and  shaping  the  peaks  and  pinnacles  of  rock  is 
very  great. 

The  term  slide-rock  implies  any  loose  fragmental  rock  lying  on  a  slope, 
while  talus  is  restricted  to  those  cases  where  there  is  a  projecting  mass,  or 
cliff,  of  country  rock  above  from  which  the  debris  has  evidently  been  derived. 


Fig.  12.  —  Rock  disintegration  and  weathering  in  high  altitudes,  with  formation  of 
long  talus  slopes  of  slide  rock.    Mt.  Sneffels,  Colorado.    W.  Cross,  U.  S.  Geol.  Surv. 

A  talus  should  not  be  conceived  as  having  a  section  like  that  of  abc  in  Fig.  13, 
a  case  which  can  rarely  happen,  but  rather  like  that  of  a'b'c1 '.  A  talus  indeed 
is  often  only  a  rather  thin  sheet  of  fragmental  material  resting  on  sloping  bed- 
rock, which  may  here  and  there  project 
through  it.  As  the  destruction  of  the  cliff 
a'  goes  on  it  may  retreat  until  its  contour 
is  like  that  of  b'c'  beneath  the  talus.  It 
may  also  be  covered  with  the  rising  talus 
until  the  latter  forms  the  whole  slope  of 


Fig.  13.  —  Section  through  a  cliff  the  mountain.     Ordinarily  coarse  material, 

and  its  talus.  blocks  of  rock,  is  seen  at  the  top  of  the 

talus   slope;    as   this  breaks   up   into  finer 

it  is  washed  down,  descends,  and  may  gradually  assume  a  more  gentle 
slope;  this  lower  part  is  frequently  made  of  soil  with  vegetation  grow- 
ing on  it.  In  warm  regions  it  is  chiefly  the  expansion  and  contraction  which 
breaks  the  bed-rock  and  forms  the  talus ;  in  cold  countries  the  action  of 
frost  is  more  important. 

Chemical  Work  in  Weathering.  —  The  mechanical  breaking  up 
of  rock  and  its  conversion  into  soil  is  powerfully  aided  by  chemical 
processes.  In  these  water,  oxygen,  and  carbon  dioxide  are  the 


24 


THE  ATMOSPHERE  AND   ITS  WORK 


Fig.  14.  —  Talus  cone  at  the  foot  of  a  gulch.  Foot  of  talus  to  mountain  top  is  3000 
feet  in  vertical  height  and  over  a  mile  in  distance.  Mt.^Etna,  Colorado.  R.  D. 
Crawford,  Geol.  Surv.  of  Colo. 


, 


THE    ATMOSPHERE    AND    ITS    WORK  25 

chief  agents.  By  them  the  chemical  compounds  forming  the  min- 
erals of  the  rocks  are  attacked  and  in  a  great  measure  changed  into 
new  substances.  The  oxygen  converts  those  of  a  lower  state  of 
oxidation  into  ones  of  a  higher;  water,  besides  being  a  solvent, 
enters  into  combination  in  many  new  compounds;  carbon  dioxide, 
with  the  water,  helps  to  bring  substances  into  solution  and  to  change 
them  into  carbonates.  Thus  in  a  general  way  we  may  say  that  as 
a  result  the  new  minerals  formed  in  the  place  of  the  old  ones  are 
more  highly  oxidized  and  contain  water  or  carbonic  acid. 

This  may  be  illustrated  as  follows:  One  of  the  most  important  of  the 
rock-making  minerals  is  feldspar*  of  which  there  are  several  varieties;  one 
of  these  known  as  orthoclase  consists  of  oxides  of  silica,  alumina,  and  potash. 
When  this  is  attacked  by  water  containing  carbon  dioxide  in  solution  the 
following  reaction  takes  place. 

Orthoclase  +  Water  +  Garb.  diox.  yields  Kaolin  +  Quartz  +  Potas.  Garb. 
2  KAlSi308    +    2   H20    +    C02       =    H4Al2Si2Og  +  4  8iOa  +   K2CO3 

'his  is  one  of  the  most  important  reactions  which  takes  place  in  nature,  since 
the  existence  of  life  is  largely  dependent  upon  it.  Animal  life  depends  on  vege- 
table life,  and  the  latter  upon  the  soil ;  kaolin  —  common^  called  clay  —  is  an 
essential  ingredient  of  good  soils,  while  carbonate  of  potash  is  a  necessary 
food  of  plant  life;  by  this  process  the  potash  in  the  rocks  is  removed  from 
the  feldspar,  converted  into  a  soluble  form,  and  vegetation  is  able  to  assimi- 
late it. 

When  the  component  minerals  of  a  rock  are  changed  by  chemical  actions 
into  new  minerals,  as  orthoclase  into  kaolin  and  quartz,  the  process  is  spoken 
of  as  decomposition,  and  it  may  be  contrasted  with  the  disintegration,  pre- 
viously mentioned,  in  which  rocks  are  mechanically  crumbled  without  chemi- 
cal change.  Generally,  both  processes  work  together. 

Solvent  Action  of  Carbonic  Acid.  —  Some  rocks,  such  as  lime- 
stone, are  composed  almost  entirely  of  calcium  carbonate,  CaC03, 
which  forms  the  mineral  known  as  calcite,  while  in  others  this  sub- 
stance acts  as  a  cement  to  bind  the  grains  to  one  another,  as  in  some 
sandstones  which  are  made  of  grains  of  quartz  sand  thus  held  to- 
gether. Calcium  carbonate  is  nearly  insoluble  in  pure  water,  but 
when  carbon  dioxide  gas,  C02,  is  dissolved  in  it  there  is  formed  an 
aqueous  solution  of  carbonic  acid,  H2C03.  This  attacks  the  cal- 
cium carbonate  and  converts  it  into  calcium  bicarbonate, 
H2Ca(C03)2,  which  is  quite  soluble  in  water.  The  natural  surface 
waters,  like  rain,  contain  more  or  less  carbonic  acid  in  solution  and 
more  is  supplied  by  decaying  vegetation ;  through  its  action  the  bind- 
ing material  is  dissolved,  the  grains  loosen,  and  the  rock  crumbles 
and  breaks  down  into  soil.  In  the  case  of  limestone  the  greater 


*  See  Appendix  A. 


26 


THE  ATMOSPHERE   AND  ITS  WORK 


Fig.  15.  —  Illustrates  the  formation  of  soil  in  place  by  rock  weathering  and  decay. 
The  material  graduates  from  firm  rock  below,  through  rotten  rock  and  then  sub- 
soil, to  true  soil  above.  The  transition  is  gradual  without  break.  The  true  soil 
above  is  colored  dark  by  decayed  organic  matter.  G.  P.  Merrill,  U.  S.  Nat.  Mus. 


THE    ATMOSPHERE    AND    ITS    WORK 


27 


part  of  the  rock  may  go  into  solution  and  be  carried  away,  leaving 
only  the  insoluble  impurities,  usually  clay,  to  remain  behind  as 
the  resultant  soil.  This  solvent  action  of  carbonic  acid  on  car- 
bonates is  one  of  great  geological  importance;  we  are  here  only 
concerned  with  it  in  so  far  as  it  helps  to  make  soil ;  what  it  accom- 
plishes in  other  ways  will  be  treated  in  a  later  place. 

Soil  in  Situ.  —  The  process  of  weathering  is  superficial  and  is 
very  slow ;  if  the  bed-rock  were  perfectly  firm,  solid,  and  continuous 
it  would  gradually  cease,  since  the  underlying  rock  would  be  pro- 
tected by  the  mantle  of  soil  upon  it.  It  is  a  common  thing,  how- 
ever, for  this  mantle  to  be  removed  as  fast  as  formed,  by  agencies 
which  will  presently  be  described,  exposing  fresh  surfaces  to  attack. 
Even  where  this  does  not  happen,  the  agents  of  weathering  may  be 
able  to  penetrate  quite  deeply  on  account  of  the  fissured  and  cracked 
condition  of  rocks,  previously  mentioned,  and  form  considerable 
depths  of  soil.  Where  this  has  taken  place,  if  one  examines  down- 
ward, as  in  wells  or  road  cuttings,  one  finds  that  the  soil  at  the  top, 
supporting  vegetation,  gradually  passes  into  a  more  or  less  coarse, 
gravelly  material  full  of  angular  bits  of  rotten  rock;  this  is  known 
as  the  sub-soil.  The  latter  passes  downward  imperceptibly  into 
decayed  rock  which  crumbles  more  or  less  easily  and  this  in  the 
same  gradual  way  into  the  firm  solid,  unaltered  bed-rock.  Thus 
there  is  a  gradual  transition  from  soil  above  to  rock  below  and 
this  proves  that  the  soil  has  been  formed  in  the  place  where  it  now 
lies  by  the  decomposition  of  the  local  rock.  Thus  these  changes 
can  be  conveniently  divided  into  the  four  stages  mentioned:  a,  soil; 
b,  sub-soil;  c,  altered  rock;  d,  unchanged  rock.  When  the  soil  lies 
where  it  has  been  made  it  is  termed  "in  place"  or  soil  in  situ.  An 
illustration  of  this  gradual  change  from  rock  below  to  soil  above 
may  be  seen  in  Fig.  15. 

The  reason  for  making  this  distinction  as  to  whether  a  soil  has  been 
formed  in  situ,  or  not,  is  important  because  over  wide  areas  the  soils  are  not 
in  place,  but  have  been  brought  from  elsewhere,  or  shifted,  by  the  action  of 
the  wind,  running  water,  moving  ice,  etc.,  as  illustrated  in  Fig.  16.  Over 
much  of  the  northern  United  States,  Canada,  and  northern  regions  generally, 


Soil  in  place 


Colin, 


'al  Soil 


Fig.  16.  —  Diagram  illustrating  the  forming  and  movement  of  soil. 


28  TEXT-BOOK   OF   GEOLOGY 

it  will  be  found  that  there  is  no  gradual  transition  from  soil  to  rock,  such  as 
described  above,  but  that  the  soil  rests  directly  upon  unchanged  solid  bed- 
rock. This  is  a  clear  proof  that  the  soil  has  been  shifted.  These  regions  were 
once  covered,  as  will  be  shown  later,  by  vast  areas  of  moving  glacial  ice, 
which  ground  away  the  rotten  rock  and  shifted  the  soil.  In  the  southern 
states  and  in  warm  and  tropical  countries  much  of  the  soil  is  in  situ,  and  in 
some  places  it  is  as  much  as  several  hundred  feet  deep. 

Kinds  of  Soil.  —  The  nature  of  the  soil  produced  by  the  decay 
of  rocks  depends  chiefly  upon  the  kinds  of  mineral  grains  of  which 
they  are  composed.  The  more  important  of  these  minerals  are  feld- 
spar, quartz,  calcite  and  clay,  which  have  been  mentioned  previously 
and  their  compositions  given.  Feldspar  changes  to  clay,  whereas 
quartz,  Si02,  is  not  affected  by  weathering.  Calcite,  CaC03,  is 

soluble  under  the  conditions 
which  have  been  stated.  In 
addition  to  these  there  are 
many  other  kinds  of  min- 
erals, such  as  the  silicates  of 
iron  and  magnesia  forming 
substances  like  hornblende, 
dark  mica,  etc.,  but  these 
are  of  lesser  importance. 
Thus  when  rocks  like  gran- 
ite, which  is  chiefly  com- 
posed of  a  mixture  of  quartz 
and  feldspar,  are  thoroughly 
decomposed  into  soil  the  lat- 
ter consists  of  clay  inter- 
Fig.  17. —  Residual  bowlder  resting  on  bed-  mingled  with  quartz  grains, 
rock,  "Balanced  Rock,"  Garden  of  the  Gods,  A  pure  feldspar  rock  Would 
Colorado.  .  ,  , 

yield  only  clay ;  while  a  pure 

sandstone  would  give  only  sand  by  disintegration.  According  to 
the  size  of  the  particles  which  compose  the  broken  and  disintegrated 
rock  the  following  gradations  are  recognized:  Pieces  of  loose  rock 
from  the  size  of  a  small  melon  up  are  termed  bowlders;  those  larger 
than  peas  are  called  gravel.  Pieces  smaller  than  peas,  but  which  do 
not  cohere  when  wet,  are  sand,  while  the  finest  material,  which  can 
be  carried  by  the  wind,  is  dust,  and  this  generally. coheres  when  wet 
and  is  termed  silt,  or  mud  or  clay,  according  to  its  character. 
Ordinary  soils  are  composed  of  variable  mixtures  of  sand,  and  these 
finer  materials.  We  may  roughly  classify  them  into  the  following 
groups: 


THE    ATMOSPHERE    AND    ITS    WORK  29 

Sand,  composed  of  sand  grains,  mostly  quartz,  without  clay. 

Loam,  mixtures  of  sand  and  clay. 

Clay,  the  finest  material,  mostly  kaolin,  without  sand. 


Of  these  loam  is  most  easily  worked  and  makes  the  best  soil;  clay 
is  next,  but  is  apt  to  be  stiff  and  difficult  to  work,  while  sandy  soils 
are  usually  light  and  also  poor  for  the  growth  of  vegetation. 

The  red  and  yellow  colors  which  many  soils  possess  are  due  to  the 
hydrous  oxides  of  iron  produced  by  the  decay  and  oxidation  of  the 
original  minerals  in  the  rocks  consisting  of  iron  oxides  and  silicates. 
A  dark  or  black  color,  best  seen  in  swampy  soils,  is  due  to  carbon- 
aceous material,  resulting  from  the  decay  of  vegetation.  This  sub- 
stance, which  is  present  to  some  degree  in  most  arable  soils,  is 
known  as  humus  and  those  very  rich  in  it  are  called  muck.  When 
a  soil  contains  a  considerable  quantity  of  carbonate  of  lime  it  is 
termed  a  marl.  Thus  sands,  loams,  clays,  mucks,  and  marls  are  the 
chief  kinds  of  soils  and  there  are  all  gradations  of  these  into  one 
another.  Owing  to  the  presence  of  the  dark  organic  matter,  or  to 
the  greater  oxidation  of  the  iron  compounds,  and  often  to  other 


Fig.  18.  —  Residual  bowlders  left  by  decomposition  and  wearing  away  of  bed-rock. 
The  bowlders  are  included  masses  of  a  harder,  more  resistant  material  and  of  rounded 
shapes  (concretions).  This  shows  that  residual  bowlders  may  in  some  cases  differ 
from  the  bed-rock  on  which  they  lie.  Coalinga,  Cal.  R.  Arnold,  U.  S.  Geol.  Surv. 


30  TEXT-BOOK    OF    GEOLOGY 

reasons  the  top  soil  is  apt  to  be  much  more  strongly  colored  than 
the  underlying  subsoil. 

Bowlders  of  Decomposition.  —  The  change  of  bed-rock  into  soil 
is  not  apt  to  take  place  equally,  either  over  small  or  wide  areas. 
For  the  rock  mass  undergoing  decomposition  is  not  everywhere  uni- 
formly cracked  and  fissured,  allowing  free  access  to  all  its  parts  of 
the  agents  of  disintegration  and  decomposition  which  have  been 
described.  Moreover,  some  parts  of  the  rock  mass  may  be  different 
in  composition  and  texture  from  the  rest,  and  thus  harder,  denser, 
more  durable,  or  less  soluble.  From  this  it  commonly  happens  that 
the  soil  is  more  or  less  filled  with  pieces  of  unchanged  or  but  little 
altered  rock,  which  are  termed  bowlders.  That  such  residual 
bowlders  have  been  formed  in  the  place  where  they  are  now  is 
proved  by  their  similarity  in  mineral  composition  and  appearance 
with  the  still  unchanged  bed-rock  below.  But  it  frequently  happens 
that  bowlders  are  very  different  in  character  from  the  rock  below 
them  and  this  shows  in  most  cases  that  they  have  been  moved  or 
are  transported  blocks.  It  may  happen  that,  where  residual  bowl- 
ders of  decomposition  are  forming,  the  soil  about  them  may  be 
removed  by  wind  or  rain  wash  as  fast  as  formed,  as  illustrated  in 
Fig.  17.  In  this  case  they  may  be  left  upon  the  surface  as  scattered 
blocks,  see  Fig.  18. 


CHAPTER  II 
RAIN  AND  RUNNING  WATER 

The  Rainfall.  —  The  amount  of  rainfall  which  a  country  re- 
ceives is  dependent  on  a  variety  of  factors,  such  as  the  direction  of 
the  prevailing  winds ;  the  nature  of  the  places  over  which  they  have 
previously  passed,  as  to  whether  these  are  land  or  water  areas,  and, 
if  the  former,  low  land,  or  high  mountainous  tracts;  the  surface 
character  of  the  country  which  receives  them,  whether  high  or  low; 
and  on  other  considerations  as  well.  Thus  it  happens  that  the 
amount  of  rainfall  received  by  the  land  is  very  unequally  distributed 
over  the  world;  in  some  places,  as  in  Central  America,  it  may  be  as 
much  as  100  inches  per  year,  while  in  the  great  deserts  it  is  less 
than  10.  In  general,  in  North  America,  one  may  say  that  in  the 
Atlantic  sea-board  region,  and  in  the  Southern  States,  the  rainfall 
is  40  inches  or  more  per  year;  as  one  goes  westward  into  the  Mis- 
sissippi valley  it  diminishes  to  30  inches  or  somewhat  more;  in  the 
Plains  region  to  20  or  less;  in  the  Great  Basin  between  the  Rocky 
Mountains  and  the  Sierras  to  10  or  less.  Locally  in  the  mountains 
it  is  increased,  as  these  are  great  condensers  of  moisture.  On  the 
Pacific  coast  it  increases  again.  Roughly  speaking  one  may  term 
those  regions  where  the  rainfall  is  20  inches  or  less  semi-arid  to 
arid,  those  where  it  is  greater  than  this  humid.  As  we  shall  see  later, 
the  work  of  geologic  processes,  and  the  results  of  this  work,  are  in 
many  ways  strikingly  different  in  arid  regions  from  those  in 
humid  ones. 

The  Run-off.  —  A  part  of  the  water  which  falls  in  rain  is  evapo- 
rated and  passes  back  into  the  atmosphere ;  another  part  sinks  into 
the  soil  and  the  fissured  or  channeled  bed-rock  below  it,  and  there, 
on  its  way  underground  to  the  sea,  becomes  an  internal  geologic 
agency  whose  work  we  shall  study  later.  Some  of  this  water, 
which  thus  sinks  at  first,  reappears  as  springs  and,  joined  by 
that  which  runs  directly  upon  the  surface,  finds  its  way  by  means  of 
rivers  into  the  sea.  This  water  which  the  streams  carry  away  from 
the  surface  of  the  land  is  known  as  the  run-off.  The  immediate 
part  of  the  run-off  is  that  which  causes  floods  and  freshets,  while  the 
springs  furnish  the  steady  supply.  It  has  been  estimated  that 

31 


32  TEXT-BOOK    OF    GEOLOGY 

there  falls  annually  upon  the  land  areas  of  the  globe  about  29,000 
cubic  miles  of  rain  water,  and  of  this  about  6500  cubic  miles  con- 
stitutes the  run-off.  In  the  Mississippi  basin  one  quarter  of  the 
total  rainfall  forms  the  run-off.  It  is  the  object  of  this  chapter  to 
describe  the  geological  work  performed  by  this  run-off. 

Movement  of  Soil  Mantle:  Erosion.  —  As  has  already  been 
shown,  the  surface  of  the  land  is,  in  general,  covered  by  a  mantle  of 
soil  resting  on  bed-rock.  Now  by  the  action  of  running  water, 
frost,  etc.,  aided  by  gravity,  this  mantle  of  soil  and  crumbled  rock, 
which,  as  it  ordinarily  appears  to  us,  seems  to  be  at  rest,  is  actually 
in  motion,  considered  from  the  geological  standpoint,  and  is  being 
continually  urged  downward  into  the  sea,  its  ultimate  goal.  On 
steep  mountain  slopes  it  goes  more  rapidly,  in  valleys  more  slowly, 
while  in  level  plains,  like  water  in  a  lake,  it  is  temporarily  at  rest. 
Its  rate  of  motion  varies  from  time  to  time  and  from  place  to  place. 
Much  of  this  motion  is  known  as  its  creep.  As  it  moves  away,  its 
place  is  supplied  by  fresh  products  of  rock  decay,  which  also  move 
in  their  turn,  and  so  the  process  is  continued,  year  in,  year  out. 
This  formation  of  rock  debris  and  its  removal  produce  the  waste  of 
the  land  surface  and  this  wasting  is  known  under  the  general  term 
of  erosion.  We  may  study  in  detail  the  various  features  of  the 
process  and  the  agents  which  perform  it;  chiefly  they  are  wind, 
running  water,  the  action  of  waves  and  moving  ice ;  the  work  of  the 
wind  has  been  already  treated;  that  of  ice  and  the  waves  will  be 
considered  later;  the  work  of  rain  and  running  water  is  to  be  con- 
sidered here. 

Rain  Wash.  —  It  is  a  well-known  fact  that  the  wash  of  the  rain 
continually  carries  away  the  soil,  moving  it  from  higher  to  lower 
places.  The  rain  drops  run  together  and  form  rills;  these  dig  out 
gullies ;  the  gullies  run  together  and  the  larger  volume  of  water  ex- 
cavates ravines  or  gulches.  These  effects  are  conspicuously  seen  on 
steep  slopes  of  soft  material,  such  as  clay,  and  are  illustrated  in 
Fig.  19.  The  result  of  this  removal  of  material  is  seen  after  every 
storm  in  the  volume  of  muddy  water  pouring  out  of  each  gully  and 
ravine  into  the  larger  channels  below.  The  amount  removed  in  a 
given  time  by  this  means  varies  greatly  with  a  number  of  circum- 
stances. For  instance  it  varies  with  the  character  of  the  soil  and 
of  the  bed-rock.  In  New  England,  for  example,  the  bed-rock  is 
hard  and  crystalline,  the  soils  stony  and  clayey  glacial  deposits 
which  resist  erosion  well;  what  the  rains  wash  into  the  streams  is 
almost  entirely  from  the  soil,  while  over  wide  areas  of  the  Southern 
States  and  the  Western  Plains,  the  country  rock  consists  of  soft, 


RAIN    AND    RUNNING    WATER  33 

little  compacted  deposits  of  sand  and  clay,  whose  resistance  to  rain 
wash  is  not  much  greater  than  that  of  the  soil  itself,  or  the  soil  is 
loose  and  deep  and  held  only  by  the  vegetation.  Rivers  draining 
these  latter  regions  are  constantly  turbid  and  filled  with  muddy  sed- 
iment. Another  feature  which  has  a  great  effect  upon  the  rate  of 
erosion,  as  partly  noted  above,  is  whether  the  soil  is  covered  with 
vegetation  or  not,  and  this  is  so  important  that  it  deserves  especial 
consideration. 

r 

Rain  wash  is  really  the  beginning  of  stream  erosion  and  should  be  consid- 
ered a  part  of  that  process,  which  is  treated  in  detail  later.  It  is  introduced 
here  because  it  is  that  phase  of  erosion  which  is  most  familiar  to  all,  and 
may  therefore  well  serve  as  an  introduction  to  the  work  of  running  water. 


Fig.  19.  —  Effect  of  rain  wash  in  beds  of  clay.     Sioux  Co.,  Neb.     N.  H.  Darton, 

U.  S.  Geol.  Surv. 

Effect  of  Vegetation  on  Erosion.  —  Where  the  soil  supports  a 
rank  growth  of  vegetation,  and  especially  if  it  has  a  thick  forest 
cover,  erosion  by  rain  wash  and  gullying  is  greatly  hindered  and  it 
may  be  almost  entirely  prevented.  There  are  two  reasons  for  this: 
first,  because  the  mass  of  roots  distributed  through  the  soil,  to- 
gether with  the  mat  of  organic  matter  on  the  surface,  holds  the  soil 
firmly  in  place  and  enables  it  to  resist  the  pressure  of  the  moving 
water,  and,  second,  because  the  mat  of  vegetation  acting  like  a 
sponge  absorbs  the  water  and  permits  it  to  drain  off  so  slowly  that 
the  destructively  erosive  effect  of  sudden  rushes  of  water  after 
storms  is  prevented.  Likewise  in  springtime  the  rapid  melting  of 
the  snow  is  hindered  by  the  forest  shade,  especially  when  it  is  com- 
posed of  evergreen  trees.  Such  effects  are  of  course  most  noticeable 


34  TEXT-BOOK    OF    GEOLOGY 

on  steep  slopes,  among  the  hills  and  mountains.  If,  in  the  settle- 
ment and  cultivation  of  a  country,  the  forest  cover  is  entirely  re- 
moved from  such  places,  erosion  starts  at  once  and  proceeds  rapidly, 
as  illustrated  in  Fig.  20,  and  in  a  variety  of  ways  great  damage  may 
be  done.  It  is  a  noticeable  fact  that  in  forest- covered  countries  the 
flow  of  the  streams  is  quite  regular  and  their  waters  relatively  clear; 
in  those  well  cleared  of  forests  and  cultivated,  the  rivers  on  the 
other  hand  are  subject,  especially  in  the  spring,  to  sudden  and  heavy 
floods,  their  waters  are  very  muddy,  and  they  are  apt  to  be  very 
low,  or  even  dry,  in  months  of  little  rainfall. 

The  regulative  action  of  the  forests  on  erosion  and  the  flow  of  rivers  is  a 
matter  of  great  importance,  not  only  from  the  geologic  standpoint,  but  as 


Fig.  20.  —  After  the  removal  of  the  forest  cover  the  soil  has  been  carried  away  so 
rapidly  that  the  remaining  trees  have  their  roots  expose* 
surface.     Southern  Appalachians.     U.  S.  Forest  Service. 


rapidly  that  the  remaining  trees  have  their  roots  exposed  by  the  lowering  of  the 


vitally  affecting  the  economic  conditions  of  civilization.  In  some  countries, 
of  which  Spain  and  northern  China  might  be  selected  as  examples,  the  im- 
provident removal  of  the  entire  forest  cover  has  reduced  large  areas,  through 
displacement  and  loss  of  arable  soil  by  erosion,  to  sterile  wastes,  subjected 
alternately  to  hot  and  baking  droughts  and  sudden  disastrous  floods.  De- 
struction of  the  forests  by  fire  may  have  a  similar  effect.  Once  destroyed, 
and  the  soil  washed  out,  they  may  not  be  restored,  or  only  with  great  diffi- 
culty after  long  periods  of  time.  Considerable  areas  in  the  Southern  States 
have  been  much  impoverished  in  this  way.  In  places,  where  density  of  pop- 
ulation causes  all  land  that  can  be  cultivated  to  be  valuable,  terracing  of  hill 
slopes  to  prevent  erosion  is  much  resorted  to.  The  yearly  loss  of  arable  soil 
is  one  of  the  great  wastes  of  modern  civilization  that  should  be  checked  as 
much  as  possible;  forests  should  be  cultivated  on  all  eminences  and  places 
not  adapted  to  agriculture  and  their  cutting  carefully  governed,  not  alone  for 
the  timber  they  may  furnish,  but  to  prevent  erosion  and  regulate  the  flow  of 
streams.  - 


RAIN    AND    RUNNING    WATER 


35 


Erosion  in  Arid  Regions.  —  In  arid  and  semi-arid  regions  the 
effects  of  erosion,  as  produced  by  rain  wash  and  gullying,  are  per- 
haps most  plainly  seen.  If  the  region  is  absolutely  rainless  there  is 
naturally  no  erosion  from  this  cause,  but  places  where  there  is  prac- 
tically no  rainfall  are  rare;  a  certain  amount  of  rain  falls  even  in 
those  districts  which  are  usually  termed  arid,  and,  as  it  is  apt  to 
come  in  heavy  and  violent  downpours,  its  effect  is  strengthened. 
The  percentage  of  run-off  is  increased  and  with  it  the  erosive  effect 
because  the  soil  contains  a  larger  quantity  of  air  in  its  pore  spaces 
than  in  humid  regions,  and  this,  when  the  surface  film  of  soil  be- 
comes moistened,  prevents  the  entrance  of  more  water.  The  lack 
of  an  adequate  mantle  of  vegetation  also  helps  the  erosive  process 
and  permits  its  effects  to  be  clearly  open  to  our  observation.  This 
also  allows  the  wind  to  perform  its  part  of  the  work,  as  described 
in  the  preceding  chapter,  in  transporting  material  from  higher  to 
lower  levels  where  it  is  more  accessible  to  the  streams  which  carry 
it  away,  or  by  dropping  it  directly  into  the  streams.  Thus  the  wind, 
which  is  of  small  importance  in  humid  regions,  becomes  a  strong 
factor  in  erosive  processes  in  arid  ones. 

Striking  examples  of  such  erosion  are  to  be  seen  along  the  rivers  which 
drain  the  Great  Plains  region.  These  rivers,  such  as  the  Missouri  and  its 


Fig.  21.  — Bad-lands,  near  Scott's  Bluff,  Neb.      N.  H.  Darton,  U.  S.  Geol.  Surv. 

tributaries,  the  Cheyenne,  Platte,  etc.,  in  places  run  in  valleys  sunk  a  con- 
siderable distance  below  the  general  level  of  the  country.  The  country  rock 
which  forms  the  sides  of  the  valleys  for  the  most  part  is  very  soft,  barely 
consolidated  clays  and  sands,  and  thus  easily  cut  by  rain  wash  and  gullying. 
The  result  is,  that  on  either  side  of  the  stream,  from  the  bottom-land  by 
the  river  to  the  bench-land  forming  the  plain,  lies  a  gradually  rising  belt  of 


36 


TEXT-BOOK    OF    GEOLOGY 


country,  cut  in  the  most  intricate  fashion  by  systems  of  gullies,  gulches,  and 
ravines,  with  spurs,  knobs,  and  sharp  ridges  separating  them  as  illustrated  in 
Fig.  21.  Such  tracts  of  country  are  known  as  Bad-lands,  from  the  difficulty 
experienced  in  traversing  them. 

It  is  to  be  noted,  that,  in  general,  where  not  merely  the  top  soil  but  the  un- 
derlying rock  is  concerned  in  erosion,  the  softer  the  material,  the  more  striking 
become  the  effects  of  gullying,  and  the  rougher  the  resultant  topography.  It 
is  as  if,  in  the  hollowing  out  of  a  trough  from  a  block  of  wood  with  a  gouge,  a 
very  soft  kind  of  wood  were  used;  then  with  each  stroke  the  hollow  chisel 
would  bite  deeply  and  the  intervening  ridges  would  be  pronounced.  In  hard 
crystalline  rocks  the  rate  of  erosion  is  dependent  on  that  of  weathering  «and 
disintegration  and  hence,  except  in  very  high  mountains  where  these  processes 
are  rapid,  the  forms  of  erosion  are  more  smooth  and  subdued. 


Remnants  of  Erosion.  —  It  has  been  explained  under  the  de- 
scription of  the  weathering  of  rocks  that  this  process  was  not  every- 
where uniform.  All  parts  are  not  equally  accessible  by  cracks  and 
fissures  to  the  agencies  which  produce  decay,  and  some  parts  may 

be  harder  and  more  resistant 
than  others.  Since  erosion  con- 

sists   in  the   loosening   of   the 

I'^^^^^^^^^lU^  rock  substance  and  its  removal, 

it  commonly  happens  in  regions 
undergoing  the  process  that, 
due  to  this  want  of  uniformity, 
there  are  projecting  masses  of 
the  more  resistant  material. 
This  is  illustrated  in  Fig.  22,  as 
occurring  on  a  small  scale,  but 
such  features  are  found  of  all 
sizes  up  to  veritable  mountains. 
Moreover,  erosion  is  less  rapid 
in  areas  between  streams,  be- 
cause the  slopes  may  be  more 
gentle  where  the  valleys  are  not 
cut  down,  and  the  material 
must  be  transported  farther; 
this  applies  especially  in  the 
level  plains  country.  Large 
isolated  masses  are  known  in  the  western  regions  as  buttes  and  are 
illustrated  in  Fig.  23 ;  they  have  been  generally  made  in  these  ways 
and  are  remnants  of  erosion.  When  such  an  elevation  is  broad  and 
flat  topped  it  is  termed  a  mesa,  from  the  Spanish,  meaning  "table" 
and  referring  to  its  table-land  character;  it  is  also  a  remnant  of 


Fig.  22.  —  Hard  masses  of  ironstone  in  beds 
of  soft  sandstone  have  shielded  the  rock 
below  them  from  erosion  and  produced 
pillars.  Monument  Park,  Colo. 


RAIN    AND    RUNNING    WATER  37 

r 


Fig.  23.  —  Red  Butte,  Bell  Ranch,  New  Mex.     W.  T.  Lee,  U.  S.  Geol.  Surv. 

erosion.  Such  mesas  are  generally  capped  by  a  layer  of  hard  rock, 
often  lava,  which  has  protected  the  softer  layers  beneath.  These 
features  and  the  rugged  sculpturing  of  mountains  all  testify  to  the 
great  work  of  erosion  and  the  amount  of  material  carried  away. 


RIVERS  AND  RIVER  VALLEYS 

Gullies  run  into  ravines  or  gulches,  and  the  rivulets  which  drain 
the  latter  run  together  to  form  brooks  and  creeks  which  in  turn 
unite  to  make  rivers.  The  rivers  then  are  the  main  channels  of 
drainage,  and  they  are  the  chief  factors  in  carrying  away  the  waste 
of  the  land.  They  are  to  be  regarded  as  the  great  trunk  lines  of 
transportation  for  the  products  of  erosion  which  are  delivered  to 
them  by  their  tributaries.  In  addition  they  are  themselves  powerful 
agents  of  erosion;  in  them  the  work  of  running  water  as  a  geological 
agency  is  most  conspicuously  displayed,  and  this  work  in  its  varied 
features  and  the  results  of  it  may  now  be  considered. 

Course  of  a  River;  its  Gradient. —  If  we  should  think  of  a  typi- 
cal river  we  should  imagine  it  rising  in  lofty  mountains  through  the 
union  of  many  impetuous  streams  or  dashing  torrents;  gathering 
headway  it  rolls  rapidly  through  the  belt  of  lower  hilly  country 
and  emerges  upon  wide  plains  through  which  it  wanders  in  many 
curves  in  a  quiet  and  steady  flow  to  the  sea.  Its  gradient,  which  may 
be  as  much  as  twenty  to  thirty  degrees  at  the  head,  becomes  less 
and  less  until  it  is  nearly  horizontal  at  the  river's  mouth,  as  illus- 
trated in  Fig.  24. 


38  TEXT-BOOK   OF   GEOLOGY 

While  we  think  of  this  as  the  ideal  course  of  a  river,  and  it  is  typical  of 
many  of  the  great  rivers  of  the  world  such  as  the  Amazon  and  the  Ganges,  and 

of  a  great  number  of  smaller  ones  of 
which  the  Po  in  Italy  might  serve  as 
an  example,  one  constantly  finds  va- 
&  nations   from   this   type.     Thus   the 

Mississippi  does  not  rise  in  a  moun- 
Fig.  24.  —  Profile  of  a  river  showing         ,    .  ,     ,    .  , 

.,  tamous  country,  but  in  a  moderately 

its  gradient.  . 

elevated  region  of  low  relief  and  it 

has  a  very  uniform  gradient  to  the  sea;  in  other  cases  where  rivers  rise  in 
mountains  near  the  sea  the  lower  plains  district,  corresponding  to  b  of  Fig.  24, 
may  be  very  short  or  wanting.  The  rivers  of  the  northern  Atlantic  coast 
are  mostly  between  these  extremes  and  their  courses  lie  between  a  and  b; 
those  of  the  Southern  States  are  more  nearly  typical,  since  they  rise  in  the 
Appalachian  Mountains  and  flow  out  upon  the  Atlantic  coastal  plain. 

River  Erosion 

Corrasion.  —  If  we  consider  the  whole  course  of  a  typical  river 
from  its  source  to  the  sea  we  find  that  it  performs  both  destructive 
and  constructive  work.  The  former  is  a  work  of  erosion  and  is  done 
chiefly  in  the  upper,  steeper  part  of  its  course,  that  portion  which  is 
indicated  by  a  in  Fig.  24 ;  we  will  examine  first  the  conditions  under 
which  this  is  carried  on,  while  the  constructive  work  will  be  treated 
later.  In  the  first  place  it  is  clear  that,  unlike  rain  wash  and  gully- 
ing, which  are  a  general  effect  over  the  whole  land  area  like  the 
result  of  weathering,  the  erosion  of  rivers  is  local  and  confined  to 
the  bottom  and  sides  of  the  channels  over  which  the  water  passes. 
A  river  may  be  compared  to  a  sinuous,  flexible  and  endless  file,  ever 
moving  forward  in  one  direction,  and  by  means  of  the  moving  sand 
or  gravel  rasping  away  the  country  rock  beneath  and  beside  it,  thus 
cutting  an  ever-deepening  trench.  This  particular  phase  of  a  river's 
work  is  called  corrasion,  and  is  one  special  form  of  general  erosion. 
The  effectiveness  with  which  a  river  corrades  depends  on  several 
closely  related  things ;  on  the  tools  with  which  the  river  has  to  work, 
on  the  amount  of  them,  on  the  swiftness  of  its  current,  and  on  the 
nature  of  the  country  rock  with  which  it  has  to  deal.  These  various 
factors  may  be  examined  in  detail. 

As  we  have  seen,  the  lowering  of  the  land  by  decay  or  wear  of  its  rock 
surfaces  and  the  removal  of  the  loosened  material  form  what  is  known  as 
erosion.  So  far  as  we  are  here  concerned  the  loosening  is  done  by  weathering 
and  corrasion,  and  the  removal  of  the  debris  by  running  water,  which  trans- 
ports it.  Rain  wash  is  chiefly  transportation  of  little  coherent  material,  but 
as  the  rills  flowing  together  grow  into  streams  corrasion  begins,  and  corre- 
spondingly increases.  Thus  we  have  weathering,  corrasion,  and  transportation, 
as  the  three  phases  of  general  erosion. 


RAIN    AND    RUNNING    WATER 


39 


The  River's  Tools.  —  Clear  water  moving  over  rock  surfaces 
erodes  but  very  little.  It  has  a  certain  solvent  power  and  may 
thus  slowly  dissolve  and  disintegrate  rocks,  and  in  this  action  can  be 
aided  by  substances  carried  in  solution,  especially  carbonic  acid. 
In  this  case  the  rock  which  is  chiefly  attacked  is  limestone,  composed 
of  carbonate  of  lime,  and  for  reasons  which  have  been  previously 
explained  (p.  25).  This  is,  however,  a  chemical  process,  rather  than 
mechanical  erosion,  or  corrasion.  In  order  to  corrade,a  river  must 
have  tools,  and  these  are  supplied  by  the  sand  and  silt  which  it 
carries,  and  by  the  gravel  and  pebbles  it  can  move  if  swift  enough, 
either  in  its  regular  flow,  or  in  times  of  flood,  see  Fig.  25.  This  ma- 
terial is  supplied  to  the  river  chiefly  by  rain  wash  and  by  its  tribu- 
taries, but  in  regions  of  soft  material  the  stream  may  also  obtain  it 
directly  by  wearing  and  undermining  the  sides  of  its  channel.  If  its 
banks  are  steep,  or  even  cliff-like,  the  natural  talus  which  would 
form  at  the  foot  of  such  a  slope  is  seized  by  the  river  and  carried 
away  to  be  used  as  its  tools.  It  is  by  the  striking,  bumping,  and 
grinding  action  of  this  material,  carried  along  by  the  current,  that 
the  river  is  able  to  cut  away  the  rocks  over  which  it  runs  and  to 
deepen  its  channel. 


Fig.  25.  —  River  bed  full  of  more  or  less  rounded  bowlders  showing  the  tools  with 
which  the  stream  works.  Big  Creek,  Hay  wood  Co.,  N.  C.  A.  Keith,  U.  S. 
Geol.  Surv. 


40  TEXT-BOOK    OF    GEOLOGY 

In  this  process  the  material  carried  by  the  river  is  itself  necessarily  worn, 
has  its  sharp  angles  removed  and  becomes  rounded  or  spheroidal,  —  a  form 
characteristic  of  the  river's  tools.  Thus,  if  we  find  river  gravels  which  con- 
sist of  hard,  well  rounded  pebbles,  we  infer  the  material  has  been  trans- 
ported a  long  distance;  on  the  other  hand  if  the  gravel  is  composed  of  angu- 
lar bits  of  rock,  and  its  situation  shows  that  it  has  been  transported,  we  infer 
that  the  distance  must  have  been  short. 

Amount  and  Size  of  Material  Carried.  —  Up  to  a  certain  point 
an  increase  in  the  amount  of  grinding  material  supplied  to  a  river 
with  a  given  velocity  of  current  aids  in  its  erosive  power.  Beyond 
this  point  an  increase  is  not  effective  for  the  reason  that  the  strength 
of  the  current  is  so  consumed  in  the  operation  of  transporting  that 
the  check,  given  by  a  tendency  to  erode,  would  cause  the  river  to 
deposit  instead.  This  is  further  discussed  under  transportation. 
Since  the  eroding  power  depends  on  the  strength  of  the  blow  with 
which  the  moving  particles  strike,  it  is  clear  that  this  in  turn  de- 
pends upon  the  momentum,  that  is,  upon  their  mass  multiplied  by 
the  velocity.  Hence  for  a  constant  velocity  the  greater  the  mass  of 
the  particles,  that  is,  the  larger  and  heavier  they  may  be,  the  more 
effective  agents  of  erosion  they  become.  Thus  in  a  stream  carrying 
intermingled  grains  of  sand  and  dust-like  particles  of  clay  the  sand 
is  the  really  effective  agent. 

Swiftness;  Law  of  Corrasive  Power.  —  It  is  quite  obvious  from 
the  preceding  paragraph  that  other  things  being  equal,  the  swifter  a 
current  is  the  more  rapidly  it  will  corrade.  For,  in  a  given  time,  not 
only  will  the  number  of  corrading  particles  passing  over  a  rock  sur- 
face be  increased  with  a  swifter  current,  but  the  fact  that  each  par- 
ticle is  moving  more  rapidly  will  aid  its  effectiveness.  This  may  be 
formulated  as  a  law  in  definite  form  as  follows:  The  corrasive  power 
of  a  current  varies  as  the  square  of  the  velocity,  with  equal  size  and  dis- 
tribution of  particles.  That  this  is  true  may  be  easily  proved.  If 
we  think  of  an  obstacle  in  the  stream,  such  for  instance  as  the  abut- 
ment of  a  bridge,  which  is  being  corraded  by  the  impact  of  sand  grains 
moving  by  the  current  and  imagine  the  velocity  to  be  doubled,  then 
on  a  given  surface,  for  each  moment  of  time,  twice  as  many  sand 
grains  will  strike  as  before  and  each  with  a  velocity  twice  as  great. 
The  effect  will  therefore  be  four  times  as  great.  If  the  velocity  is 
three  times  as  great,  three  times  as  many  grains  will  strike,  each 
with  a  trebled  velocity,  and  the  effect  will  be  nine  times  as  great. 
Therefore  the  effect  varies  in  this  case  as  the  square  of  the  velocity. 

While  this  is  true  in  theory  the  swifter  stream  will,  however,  carry 
larger  particles,  which,  owing  to  their  momentum,  strike  with  greater  force. 


RAIN  AND  RUNNING  WATER  41 

The  actual  corrasive  power  varies  between  the  square  and  sixth  power  of  the 
velocity,  as  may  be  understood  after  "velocity  and  transportation"  beyond  has 
been  read. 

Character  of  the  Country  Rock.  —  It  is  quite  evident  that  if  a 
stream  passes  through  a  region  whose  underlying  rock  masses  are 
relatively  soft  or  of  little  coherence  it  will  erode  much  more  rapidly 
than  in  a  region  of  very  durable  rocks.  This  of  course  applies  as 
well  to  general  erosion  as  to  those  particular  surfaces  affected  by 
the  moving  water  of  the  stream  alone.  Examples  of  this,  as  pre- 
viously stated,  are  seen  in  the  rivers  of  New  England  and  eastern 
Canada,  which,  flowing  over  hard  crystalline  rocks,  are  relatively 
clear  compared  with  those  of  the  Southern  Atlantic  States  and  of  the 

reat  Plains,  which  pass  through  regions  of  soft  material  and  are 
bid  with  the  products  of  rapid  erosion. 


ai 

!  '* 


aL-^ 


Fig.  26.  —  Erosion  in  horizontal  beds.  Fig.  27.  —  Erosion  in  inclined  beds. 

The  structure  of  the  rock  masses  has  also  much  to  do  with  the  rapidity  of 
erosion.  Thus  if  they  are  greatly  jointed,  that  is,  filled  with  cracks  and  fissures, 
the  progress  of  erosion  is  greatly  aided.  If  they  consist  of  alternately  hard 
and  soft  layers,  as  is  often  the  case,  the  position  of  these  layers  has  a  great 
effect  in  determining  the  rate  of  erosion.  Thus  in  Fig.  26  when  the  hard 
layer,  or  bed,  b,  is  reached  it  serves  as  a  floor  for  the  stream  and  erosion 
proceeds  slowly  until  it  is  worn  through:  in  Fig.  27  the  stream  eats  its  way 
downward  along  the  soft  beds,  a,  and  the  hard  ones,  b,  losing  their  support 
by  undermining,  break  away  in  pieces  and  are  worn  and  carried  away.  Thus, 
in  this  case,  the  rate  is  determined  by  the  soft,  not  by  the  hard  layers.  The 
rock  structure  shown  in  the  figure  could  not  remain. 

Transportation;  the  River's  Burden 

A  river  not  only  erodes,  but  also  transports  and  the  material  car- 
ried by  it  forms  its  load  or  burden.  While  the  greater  part  of  this 
is  carried  mechanically  in  suspension,  a  very  considerable  portion  is 
transported  chemically  in  solution,  while  still  another  part  is  rolled 
or  moved  along  the  bottom.  The  ultimate  goal  of  the  river  is  the 
sea  into  which,  at  the  end  of  its  journey,  its  burden  is  transferred 
and  deposited.  The  various  features  of  this  work  demand  con- 
ideration. 

Material  in  Suspension.  —  The  size  of  the  particles  that  a  river 
able  to  carry  in  suspension  depends  on  several  things;  on  the 


42  TEXT-BOOK    OF    GEOLOGY 

character  of  a  river's  current,  on  its  swiftness,  and  on  the  relative 
weight  or  specific  gravity  of  the  particles.  With  respect  to  the  first 
of  these,  if  the  mass  of  water  forming  the  current  moved  forward  in 
a  perfectly  uniform  manner,  each  particle  of  water  from  side  to  side 
and  from  top  to  bottom  moving  forward  with  the  same  velocity  as 
every  other  particle,  only  the  very  finest  material,  such  as  micro- 
scopic granules  of  clay,  would  remain  any  length  of  time  in  sus- 
pension. A  sand  grain  dropped  into  the  stream  would  sink  to  the 
bottom  and  there  remain  at  rest,  unless  the  stream  were  strong 
enough  to  roll  it  along.  But  the  current  of  streams  is  not  of  this 
character.  The  more  central  portions  are  moving  more  swiftly, 
sliding  over  those  toward  the  bottom  and  sides,  while  there  is  a  con- 
stant interweaving  of  swifter  sub-currents  up  and  down  and  toward 
the  sides  and  even  backward,  forming  eddies  or  whirling  movements. 
The  whole  effect  is  like  the  stirring  of  water  in  a  glass.  Sand  at 
the  bottom  is  quickly  lifted  and  kept  in  suspension  by  these  move- 
ments and  thus  carried  along. 

When  particles  in  suspension  in  pure  water  attain  a  certain  degree  of  fine- 
ness their  settling,  even  when  the  water  is  still,  becomes  very  slow.  Thus,  as 
shown  by  the  experiments  of  Brewer,  river  waters,  such  as  that  of  the  Mis- 
sissippi, carrying  fine  clay  particles  may  remain  turbid  for  many  years.  In 
such  cases  with  increasing  fineness  there  seems  to  be  no  limit  in  a  practical 
way  between  material  in  suspension  and  that  in  solution,  however  different 
these  may  be  in  theory.  If  such  waters  containing  clay  in  suspension  be  ren- 
dered salt  and  agitated  the  clay  curdles,  or  coagulates,  into  lumps,  and  is 
quickly  deposited,  leaving  the  liquid  clear.  'Thus  the  clay  of  river  waters,  on 
their  attaining  and  mingling  with  the  salt  water  of  the  sea,  is  deposited  on  the 
bottom. 

Velocity  and  Transportation.  —  The  velocity  of  a  current  de- 
pends, not  only  on  the  slope,  but  also  on  the  quantity  of  water,  thus 
of  two  streams  having  similar  gradients  and  form  of  channel,  the 
one  having  the  larger  amount  of  water  will  have  the  swifter  current. 
It  is  also  well  known  that  the  swifter  a  current  is  the  larger  and 
heavier  masses  it  can  transport.  Thus  it  has  been  found  that  a  cur- 
rent running  a  fifth  of  a  mile  in  an  hour  will  carry  fine  clay;  one 
running  half  a  mile  in  an  hour  will  transport  sand;  one  of  a  mile 
an  hour  will  roll  along  medium-sized  gravel,  while  one  of  two  miles 
an  hour  will  sweep  along  pebbles  the  size  of  an  egg.  Reduced  to 
mathematical  form  it  may  be  stated  that  the  moving  power  of  a 
current  varies  as  the  sixth  power  of  the  velocity.  This  seems  extraor- 
dinary but  may  be  easily  demonstrated.  Suppose  the  problem 
to  be  stated  as  follows:  if  a  current  of  a  given  velocity  is  just  able 


RAIN    AND    RUNNING    WATER 


43 


to  move  a  cube  a,  Fig.  28,  what  will  be  the  comparative  size  of  the 
cube  which  a  current  of  twice  this  velocity  can  move?  Now  it  has 
been  previously  shown  that  if  the  velocity  of  the  current  is  doubled, 
its  striking  force  is  four  times  as  great,  because  in  a  given  moment 
of  time  twice  as  much  water  strikes  on  the  cube  face  of  a  and  each 
particle  of  water  has  twice  the  velocity.  The  effect  of  the  doubled 
current  being  thus  four  tunes  as  great,  it  could  move,  striking  on 
the  cube  face  a,  four  such  cubes  a1  a2  a3  a4  placed  one  behind  the 
other  to  form  a  four-sided  prism.  If  the  current  could  move  one,  it 
could,  within  limits,  move  any  number  of  such  prisms  endwise.  But 
the  problem  was,  how  much  larger  a  cube  could  it  move?  Now  it  is 
clear  that  16  of  these  prisms  piled  together  as  in  Fig.  28  would  form 


Fig.  28.  —  Diagram  proving  that  if  the  velocity  is  doubled  the  transporting 
power  is  64  times  as  great. 

a  cube  and  the  current  striking  on  the  face  A  would  just  move  it. 
It  can  be  readily  seen,  however,  that  the  new  cube  A  is  64  times  as 
great  as  the  original  cube  a.  But  64  equals  the  sixth  power  of  two. 
If  the  velocity  of  the  current  were  trebled  then  the  striking  force 
would  be  9  times  as  great,  for  three  times  as  much  water  would 
strike  in  a  unit  of  time  with  three  times  the  velocity;  this  would 
move  a  prism  9  cubes  long  and  it  would  require  81  such  prisms  to  be 
piled  together  to  form  a  cube.  But  such  a  cube  would  be  729  times 
as  large  as  the  original  one  and  729  equals  the  sixth  power  of  three. 
Therefore  the  transporting  power  varies  as  the  sixth  power  of  the 
velocity.  This  affects  corrasion  also. 

With  low  velocities  of  less  than  a  mile  an  hour  and  with  fine  particles  this 
law  of  increase,  although  equally  applicable,  does  not  seem  very  striking.  Thus 
sand  grains  may  be  a  hundred,  or  a  thousand  fold,  as  large  as  those  of  fine  silts 
or  muds  and  require  a  doubled  or  trebled  velocity  to  move  them.  But  with 
increasing  speeds  of  miles  per  hour  the  effect  becomes  very  marked  and 
this  explains  why  rapid  streams  of  five  miles  per  hour  are  able  to  move  small 
bowlders,  while  sudden  floods  in  narrow  valleys,  caused  by  torrential  down- 
pours of  rain  or  the  bursting  of  dams,  are  able  to  carry  with  them  huge 
masses  of  earth  and  rocks,  sweep  away  bridges  and  other  structures,  and 
cause  great  damage;  see  Fig.  29. 

It  is  also  clear  that  the  less  coherent  the  material  is  in  which  a  stream  is 


44 


TEXT-BOOK    OF   GEOLOGY 


eroding  the  more  nearly  will  the  eroding  power  approach  that  of  its  full  trans- 
porting power ;  hence  the  eroding  —  and  transporting  —  will  vary  between  the 
square  and  sixth  power  of  the  velocities. 


Fig.  29.  —  In  times  of  flood  a  stream  is  able  to  carry  masses  and  perform  work  vastly 
greater  than  under  ordinary  conditions,  as  here  shown  by  the  results  of  flooding. 
Manti  Creek,  Utah.  H.  Gannett,  U.  S.  Geol.  Surv. 


Effect  of  Specific  Gravity.  —  The  size  of  the  particle  that  a 
stream  of  a  given  velocity  is  able  to  carry  depends  on  the  specific 
gravity  of  the  materials  composing  the  particle.  A  familiar  example 
of  this  is  seen  in  that  a  lead  sinker  is  able  to  remain  at  rest  on  the 
bottom  of  a  stream  which  carries  away  pebbles  of  an  equal  size.  A 
practical  application  of  importance  is  found  in  the  fact  that  in  placer 
mining,  which  consists  in  extracting  gold  from  river  sands  and 
gravels,  excessively  fine  particles  of  the  precious  metal  are  mixed 
with  vastly  larger  ones  of  the  sand  and  gravel ;  the  water,  on  account 
of  the  high  specific  gravity  of  the  gold,  being  unable  to  transport  it. 
Hence  if  the  gold  grains  are  relatively  coarse  and  angular,  that  is, 
unworn,  it  is  inferred  that  they  cannot  have  been  transported  far 
from  the  original  lodes  or  rocks  which  contained  them  and  from 
which  they  were  derived  by  erosion.  The  specific  gravity  of  the 
great  mass  of  material  obtained  by  erosion  and  carried  by  rivers 


RAIN   AND   RUNNING   WATER  45 

lies  between  2.5  and  3.0,  that  is,  it  is  that  much  heavier  than  an 
equal  volume  of  water.  It  should  also  be  remembered  that  a  body  im- 
mersed in  water  loses  weight  equal  to  that  of  the  volume  of  water 
displaced,  and  this  greatly  aids  the  transporting  power  of  the  stream. 
Transport  on  the  River  Bed ;  Traction.  —  Besides  the  material 
carried  in  suspension  a  considerable  part  of  the  river's  burden  is 
pushed,  or  rolled,  along  the  bottom.  What  proportion  of  the  whole 
this  may  be,  cannot  be  accurately  determined;  it  has  been  thought 
that  in  the  case  of  some  rivers  it  is  greater  than  the  amount  carried 
in  suspension.  From  the  studies  which  have  been  made  on  the 
Mississippi  it  is  roughly  inferred  that  of  the  material  which  it  car- 
ries into  the  Gulf  of  Mexico  about  10  per  cent  or  more  consists  of 
coarser  detritus  moved  along  the  bottom.  It  is  obvious  that,  other 
things  being  equal,  the  steeper  the  gradient  a  river  has  the  larger  will 
be  the  amount  of  the  moved  material. 

By  observation  of  experiments  in  troughs  it  has  been  found  that  of  the 
material  urged  along  the  bottoms  of  streams  the  amount  moved  by  sliding  or 
rolling  of  the  particles  is  relatively  small,  compared  with  that  which  pro- 
gresses by  a  series  of  short  leaps.  For  a  certain  distance  above  the  bottom 
there  is  a  zone  filled  with  grains  moving  in  this  fashion,  called  saltation 
(jumping),  and  above  this  is  the  material  in  suspension.  In  distinction  from 
that  carried  in  suspension,  the  particles  urged  by  rolling,  or  saltation,  are  said 
to  be  moved  by  stream  traction.  The  amount  carried  by  traction,  compared 
with  suspension,  varies  with  a  number  of  factors,  such  as  the  swiftness  of 
the  current,  size  of  the  grains,  etc.  There  is  also  a  collective  movement 
of  heaps,  or  waves,  of  sand  downstream. 

Burden  Carried  in  Solution.  —  All  river  waters  carry  in  solution 
salts  of  various  kinds  which  have  been  leached  from  the  rocks  and 
soils  of  the  country  from  which  they  drain.  While,  in  a  measured 
volume  of  what  we  call  fresh  water,  the  amount  may  seem  rela- 
tively very  small,  in  the  aggregate,  the  weight  of  material  thus  dis- 
solved from  the  land  and  carried  into  the  sea  is  enormous.  It  has 
been  estimated  that  annually  nearly  2,735,000,000  metric  tons  *  of 
solid  substances  are  thus  transported  into  the  oceans.  The  Mississippi 
carries  about  136,000,000  tons,  the  Connecticut,  a  small  river, 
1,000,000,  the  Danube  over  22,000,000,  the  Nile  nearly  17,000,000. 
In  the  Mississippi  the  amount  carried  in  solution  is  more  than  a 
third  as  large  as  that  carried  in  mechanical  suspension,  the  quanti- 
ties being 

340,500,000  tons  in  suspension; 
136,400,000  tons  in  solution. 

*  Metric  ton   =    1000  kilograms    =    2204  pounds. 


46  TEXT-BOOK    OF    GEOLOGY 

The  most  important  of  the  substances  thus  dissolved  and  trans- 
ported are  the  carbonates  of  lime  and  magnesia,  CaC03  and  MgC03 ; 
the  sulphates  of  lime,  soda  and  potash,  CaS04,  Na2S04,  and  K2S04; 
chloride  of  sodium,  NaCl,  and  silica,  Si02.  In  humid  regions  where 
there  is  much  vegetation,  the  latter  by  its  decay  generates  carbonic 
acid,  and  by  the  aid  of  this  the  percolating  waters  dissolve  lime  and 
other  carbonates  from  the  rocks,  as  previously  explained.  Hence  in 
humid  regions  the  water  of  rivers,  like  the  Potomac  and  the  Dela- 
ware, has  chiefly  carbonates  in  solution;  in  arid  regions  where 
vegetation  is  sparse  or  wanting  it  contains  mostly  sulphates  and 
chlorides,  as  in  the  Colorado  and  the  Rio  Grande. 

Estimation  of  a  River's  Burden.  —  To  ascertain  the  amount  of 
material  carried  by  a  river,  its  average  annual  discharge  of  water 
must  be  known,  and  the  average  amount  of  sediment  in  suspension 
and  of  salts  in  solution,  in  a  measured  volume,  obtained.  For  the 
former  the  area  of  the  average  cross  section  and  the  average  flow,  in 
feet  per  second,  must  be  known,  by  constantly  repeated  measure- 
ments, during  every  part  of  the  year.  The  cross  section  multiplied 
by  the  flow  gives  the  average  discharge  per  second,  from  which  the 
yearly  discharge  can  be  obtained.  In  a  similar  manner  repeated 
filtrations  of  unit  volumes  of  the  water  will  give  the  sediment  in 
suspension,  which  can  be  weighed,  while  evaporation  of  the  filtrate 
would  yield  the  salts  in  solution,  which  can  also  be  weighed.  The 
composition  of  the  salts  can  then  be  found  by  chemical  analysis. 
The  amount  moved  along  the  bottom  by  stream  traction  can  at 
present  be  only  very  roughly  estimated,  or  guessed  at. 

Many  of  the  great  rivers  of  the  world  have  been  more  or  less 
studied  in  this  way,  the  Mississippi  probably  the  most  completely, 
and  the  following  data  obtained  for  this  river  are  of  interest  and 
importance : 

Average  annual  discharge  22,000,000,000,000  cubic  feet.    • 
Average  annual  amount  in  suspension  340,500,000  tons. 
Average  annual  amount  in  solution  136,400,000  tons. 
Average  annual  amount  rolled  on  bottom,  say  40,000,000  tons. 
Total  annual  burden  516,900,000  tons. 

Rate  of  Erosion.  —  It  has  been  estimated  that  the  above  amount 
of  material,  in  suspension,  in  solution,  and  rolled  on  the  bottom,  dis- 
charged each  year  into  the  Gulf  of  Mexico,  on  the  basis  that  it 
averages  165  pounds  to  the  cubic  foot,  if  gathered  together  would 
form  a  right-angled  prism  with  a  base  one  mile  square  and  a  height 
of  250  feet.  If  we  reckon  the  whole  basin  of  the  Mississippi  and 
its  tributaries  as  covering  1,265,000  square  miles,  and  consider 


RAIN    AND    RUNNING    WATER  47 

only  the  material  in  suspension  and  solution,  it  has  been  calculated 
from  the  given  data  that  the  basin  lowers  at  the  average  rate  of  one 
foot  in  6000  years.  An  estimate  for  the  whole  United  States,  based 
on  measurements  made  on  its  rivers,  is  about  one  foot  in  9000 
years.*  These  figures  appear  too  great,  because  the  amount  moved 
by  stream  traction  is  not  included,  and  the  two  rivers,  the  Missis- 
sippi and  the  Colorado,  which  together  transport  about  80  per  cent 
of  the  total  material  taken  from  the  United  States  each  year  and 
delivered  held  in  suspension  into  the  sea,  are  also  those  which  must 
move  the  most  by  traction.  Older  estimates  for  the  Mississippi 
basin  have  been  as  low  as  one  foot  in  4000  years,  or  even  less.  The 
rate  for  its  basin  must  be  faster  than  that  of  the  United  States  as 
a  whole,  because  large  areas,  like  the  Great  Basin,  contribute  little, 
or  nothing,  to  the  annual  run-off. 

We  cannot  therefore  at  the  present  time  estimate  the  rate  of 
erosion  (denudation)  with  any  approach  to  real  accuracy,  but  the 
results  are  of  interest  and  importance  because  they  indicate  the 
order  of  magnitude  of  the  figures  concerned.  We  may  say,  with 
some  confidence,  that  the  area  of  the  United  States  is  being  lowered 
at  a  rate  of  one  foot  in  from  5000  to  10,000  years,  and  probably  be- 
tween 7000  and  9000,  and  where  thousands  of  feet  in  thickness  of 
rocks,  as  we  shall  see  later,  have  been  removed  by  erosive  processes, 
this  gives  us  some  notion  of  the  immensely  long  periods  of  time 
required  to  do  it. 

Other  rivers,  according  to  circumstances,  have  given  different  figures.  Thus 
it  has  been  calculated  that  the  Ganges  erodes  its  basin  at  the  rate  of  one  foot 
in  about  1750  years.  But  its  basin  culminates  against  the  loftiest  mountains 
in  the  world  and  the  river  has  a  proportionately  rapid  descent  and  erosive 
power.  The  basin  is  also  subject  during  part  of  the  year  to  a  very  heavy 
rainfall  and  great  floods.  Thus  the  rate  is  far  greater  than  the  average.  On 
the  other  hand  desert  regions,  like  those  in  central  Asia  or  the  Sahara  in 
Africa,  with  very  little  rainfall,  erode  with  great  slowness,  the  chief  agent  of 
transport  being  the  wind.  The  average  height  of  North  America  above  the 
sea  has  been  roughly  estimated  as  about  2000  feet;  at  the  rate  of  one  foot  in 
7500  years  it  would  take  15,000,000  years  to  reduce  it  to  sea-level,  but  as 
erosive  processes  (excepting  solution)  go  on  more  and  more  slowly  as  the 
slope  is  reduced,  this  time  in  reality  would  be,  proportionately,  enormously 
lengthened  out. 

Manner  of  Transport.  —  In  considering  the  manner  in  which 
material  is  carried  one  must  recall  that  it  is  only  in  swift  streams 
and  the  upper  rapid  tributaries  of  great  rivers  that  bowlders  and 
coarse  gravels  are  moved,  especially  in  times  of  flood.  As  one  goes 

*  Dole  and  Stabler. 


48 


TEXT-BOOK    OF    GEOLOGY 


down  a  great  river  the  size  of  the  material  steadily  grows  less  as  the 
gradient  diminishes.  This  is  seen,  not  only  in  the  matter  in  sus- 
pension, but  on  the  bars  and  beaches  where  it  is  temporarily  de- 
posited. Finally,  in  those  rivers  which  wander  through  wide 
plains  before  they  reach  the  sea,  only  the  finest  sands,  silts,  and 
clays  are  discharged  into  the  ocean,  and  no  coarse  material  is  seen, 
except  in  those  accidental  cases  where  pebbles  and  bowlders  have 
been  carried,  attached  to  masses  of  river  ice  or  entangled  in  the 
roots  of  trees,  which  float  them  downstream. 

Nor  is  the  journey  a  steady  or  uninterrupted  one.  The  gradient 
changes  from  place  to  place  and  with  it  the  velocity  and  transporting 
power.  Material  carried  down  one  reach  is  deposited  at  the  foot  of 


Fig.  30.  —  A  heavily  burdened  river.  Note  the  wide  bed  with  many  shallow  inter- 
lacing channels  and  the  very  broad,  little  cut  valley.  Compare  with  Fig.  32. 
North  Platte  River,  above  Gering,  Neb.  N.  H.  Darton,  U.  S.  Geol.  Surv. 

it,  while  at  the  head  of  the  next,  rapid  erosion  is  excavating  the  chan- 
nel upward  and  material  is  again  set  in  motion.  Matter  which  is 
dropped  at  one  time  of  year,  when  the  current  is  slack,  is  seized  and 
again  hurried  forward  with  the  renewed  strength  that  comes  in  times 
of  flood.  Thus,  with  many  waits  and  pauses,  and  growing  finer  by 
attrition,  the  mass  of  material,  upon  which  the  river  works,  is  being 
urged  forward,  more  and  more,  and  ever  onward,  down  stream. 

Graded   River.  —  Since  in  those  places  where  the  gradient  is 
lessened  a  stream  tends  to  deposit  (aggrade),  while  erosion  again 


s< 

! 


RAIN    AND    RUNNING    WATER  49 

sets  in  when  the  gradient  increases,  it  follows  that,  as  time  goes  on, 
a  river  proceeds  to  fill  up  the  hollows  and  to  cut  away  the  projec- 
tions in  its  bed  and  to  thus  establish  a  definite  gradient.  The 
gradient  which  the  river  seeks  to  establish  is  that  at  which,  in  each 
part  of  its  course,  the  velocity  is  sufficient  for  the  volume  of  water 
there  present  to  transport  its  burden  without  erosion  or  deposition ; 
it  is  then  said  to  be  graded.  This  does  not  mean  that  the  gradient  is 
necessarily  a  uniform  one  from  source  to  ^ea ;  it  may  be  relatively 
much  steeper  in  the  upper  course,  where  the  burden  is  of  coarse 
detritus  and  the  water  volume  small,  than  in  the  lower  part  where 
the  slope  is  gentle  but  the  water  volume  large  and  the  load  of  fine 
sediment.  A  very  heavily  burdened  stream,  like  the  Platte,  Fig.  30, 
may  become  graded  on  a  relatively  steep  slope,  as  compared  with  an 
underloaded  one,  which  on  such  a  slope  would  be  ungraded  and  still 
roding  (degrading) .  It  is  also  clear  from  this  that  the  lower  parts 
of  rivers,  especially  of  the  great  rivers,  such  as  the  Mississippi,  become 
graded  while,  in  the  head  waters,  cutting  and  deepening  by  erosion 
is  still  actively  going  on.  See  Fig.  32. 

Thus  in  summation  we  may  say  that  a  stream  is  at  grade,  or 
graded,  when  its  transporting  power  and  the  load  given  it  to  carry 
are  equal.  It  is  aggrading  when  the  load  it  has  to  carry  exceeds 
its  ability  to  transport.  It  is  degrading  when  its  ability  to  do  work 
is  in  excess  of  the  material  to  be  carried,  and  the  excess  of  energy 
is  employed  in  deepening  its  channel.  Grade  therefore  is  a  certain 
balanced  condition  a  river  may  attain. 

River  Valleys 

River  valleys  are  one  of  the  most  expressive  features  of  the  work 
of  erosion  by  rain  and  running  water  and  their  characters,  are  best 
seen  in  the  upper  courses  of  rivers  where  these  agencies  are  most 
actively  at  work.  The  normal  profile,  or  cross  section  of  a  valley 
which  is  undergoing  erosion  is  that  of  a  V,  as  shown  in  the  diagram, 
Fig.  31,  because  the  river,  occupying  a 
relatively  small  space,  is  cutting  down- 
ward in  the  center,  while  at  the  same 
time  rain  wash  and  gullying  tend  to 
broaden  the  trench  which  the  river 
makes,  by  washing  down  the  material  Fis-  31.  — Section  of  a  river 

.     *      .  „  ,,  .          ,          •,  valley;  ara,  material  removed 

composing  the  valley  walls.    As  already      by  corrasion.  brb  and  ^  ma_ 

shown,  as  fast  as  this  debris  reaches  the        terial  removed  by  weathering 

river,  it  is  seized  and  carried  away.    The      and    rain    wash'     river'    r' 

~,  ,  .   ,       .,  „  T      i  j  trenching  downward. 

>rofile   which   the   valley   displays   de- 


50 


TEXT-BOOK    OF   GEOLOGY 


pends  then  on  the  relative  balance  between  two  agencies, 
the  downcutting  by  the  river  and  the  broadening  by  weathering  and 
rain  wash.  Thus  in  regions  where  the  gradients  are  very  steep, 
downcutting  by  the  river  may  proceed  much  more  rapidly  than 
weathering  and  rain  wash,  and  the  valleys  will  be  deeply  incised 
and  have  profiles  approaching  brb  in  Fig.  31.  As  time  goes  on  and 
the  river  gradient  is  lessened  the  cutting  by  the  river  becomes 
slower  and  slower;  weathering  then  becomes  relatively  more  and 
more  pronounced  and  the  valley  widens  out  as  shown  in  crc  of  Fig. 
31.  A  valley,  of  the  form  shown  in  Fig.  32,  whether  or  not  it  is 
deeply  incised  below  the  general  upland,  but  where  downcutting  by 


Fig.  32.  —  A  valley  in  a  youthful  stage  of  its  history.     Yellowstone  River.     J.  P. 
Iddings,  U.  S.  Geol.  Surv. 


RAIN    AND    RUNNING    WATER  51 

the  river  predominates  over  weathering  and  rain  wash,  so  that  the 
valley  has  steep  walls,  is  called  a  young  or  immature  valley ;  one  in 
which  downcutting  has  become  very  slow  and  which  is  broadly 
opened  by  lateral  stream  cutting  and  weathering  is  termed  a  mature 
valley. 

It  should  be  clearly  understood,  in  the  use  of  these  terms,  young  and  ma- 
ture, that  absolute  age  is  not  at  all  referred  to;  that  they  are  merely  expres- 
sions to  denote  relative  stages  of  development  of  topographic  form  in  a  river 
valley  during  its  history.  Furthermore,  as  discussed  in  the  following  section, 
the  form  of  a  valley  depends  very  much  on  the  kinds  of  rocks  encountered 
by  the  river,  hence  in  one  place  the  valley  may  be  open  and  mature,  in 
another  narrow  with  rocky  walls  and  youthful,  though  the  stream  may  have 
been  running  through  both  places  the  same  length  of  time.  We  may  there- 
fore apply  the  terms  young,  mature,  and  old  to  the  corresponding  stages  of 
development  of  particular  topographic  forms,  but  not  to  a  whole  region, 
which  may  display  a  variety  of  topographic  features,  some  of  which  may 
be  much  more  advanced  than  others.  Thus  we  might  have  a  plateau, 
greatly  dissected  by  an  intricate  network  of  streams  and  their  tributaries 
running  in  deep,  narrow  valleys.  We  may  speak  of  the  plateau  as  mature, 
or  maturely  dissected,  but  the  valleys  are  yet  in  a  youthful  stage  of 
development. 

Irregularities:  Canyons  and  Gorges.  —  The  irregular  windings 
of  many  river  valleys  are  due  to  causes  which  determine  the  courses 
of  rivers  at  the  beginning  of  their  history.  If,  for  instance,  a  river 
commences  upon  a  new  land  surface,  its  course  will  be  determined 
by  the  natural  slopes  and  accidents  of  drainage  it  may  encounter ; 
as  time  passes  and  the  valley  deepens,  such  windings  give  rise  to 
the  series  of  alternating  spurs  which  characterize  most  valleys  and 
are  illustrated  in  Fig.  33.  On  the  other  hand  it  is  evident  that  in 
the  balance  between  river  deepening  and  the  widening  of  a  valley 
the  nature  of  the  material  operated  upon  must  be  a  prominent  factor. 
Few  rivers,  if  any,  flow  continuously  through  regions  of  homo- 
geneous rocks  of  uniform  resistance  to  erosion.  But  in  the  trench- 
ing of  the  river,  by  its  strong  grinding  action,  relative  degrees  of 
rock  hardness  may  have  but  little  effect,  or  none,  while  such  differ- 
ences may  produce  marked  ones  in 
valley  widening  which  is  due  to 
the  much  milder  agencies  of 
weathering  and  rain  wash.  A  mass 
of  rock  of  a  certain  kind  may  yield  Fi*-  33'  ~  C?urse  °f  a  river  valley 

with  alternating  spurs. 

readily   to   the   former   and   resist 

sturdily  the  latter.  Such  variations  in  material  produce  local  irreg- 
ularities in  the  general  form  of  valleys. 


52 


TEXT-BOOK    OF   GEOLOGY 


This  is  illustrated  in  the  case  of  many  streams  flowing  downward  from  the 
Rocky  Mountains  to  the  plains  below.  Where  they  pass  through  beds  of  soft, 
easily  eroded  shale-rock  their  valleys  are  open  and  smiling;  where  they  enter 
hard  resistant  limestone  strata  the  valley  walls  close  into  stern  and  rocky 
gorges  or  canyons. 

The  inability  of  weathering  and  rain  wash  in  widening  to  keep  pace  with 
deepening  by  river  trenching  in  resistant  material  is  well  illustrated  by  the 


Fig.  34.  —  Grand  Canyon  of  the  Colorado  River.  View  is  mostly  of  the  inner  gorge;  the 
wall  of  the  upper  broader  canyon  is  seen  in  the  distance.  J.  Hiller,  U.  S.  Geol.  Surv. 

Ausable  Chasm  in  the  Adirondacks,  a  gorge  from  100  to  200  feet  deep  and 
from  20  to  40  feet  wide,  cut  in  hard  sandstone.  Still  more  striking  examples 
are  seen  in  the  southern  Appalachian  Mountains;  as  in  the  gorge  of  the 
French  Broad  River  in  North  Carolina,  or  that  of  the  Tallulah  River  in 


ti 

I 


RAIN    AND    RUNNING    WATER  53 

Georgia,  which  is  nearly  1000  feet  deep.  Notable  examples  occur  in  Califor- 
nia on  the  streams  flowing  from  the  Sierras  and  in  many  other  parts  of  the 
world. 

In  arid  regions  the  process  of  valley  widening  through  weathering 
is  less  rapid  and  effective  than  in  humid  ones,  while  the  main  drain- 
ages, collecting  such  water  as  falls,  are  still  subjected  to  stream 
trenching.  Also,  although  storms  may  not  be  frequent,  the  rain  is 
apt  to  fall  in  heavy  downpours,  causing  strong  rushes  of  water  in 
the  drainage  channels,  with  decided  erosive  effect.  Hence  deep,  nar- 
row ravines,  or  coulees,  gorges,  or  canyons,  are  common  features  of 
topographic  relief  in  such  regions.  The  nature  of  the  rock  masses 
perated  upon  is  also  an  important  feature  in  canyon  formation, 
ince  a  narrower  valley  results  in  hard  resistant  rock  rather  than  in 
soft,  friable  one  incapable  of  maintaining  steep  walls;  if  the  rock 
beds  are  horizontal  the  condition  is  more  favorable  than  when  they 
are  inclined.  If  the  region  is  much  elevated  these  features  become 
ccentuated  because  of  the  greater  trenching  power  of  the  streams, 
owing  to  the  increased  declivity.  If  the  rivers  rise  in  an  area  of 
greater  rainfall  and  project  themselves  into  one  of  aridity  these 
effects  become  still  more  marked,  owing  to  the  increased  and  more 
constant  volume  of  water.  All  of  these  conditions  of  elevation,  rock 
structure,  aridity,  and  water  volume  are  met  in  the  rivers  which 
drain  the  Plateau  region  of  the  Southwest,  notably  the  Colorado 
River  and  its  tributaries.  Rising  in  the  Rocky  Mountains,  where 
the  precipitation  is  considerable,  they  flow  out  in  strong  volume  into 
an  elevated  region  of  proper  rock  structure,  whose  descent  affords 
steep  gradients,  and  whose  arid  climate  renders  valley  widening  ex- 
tremely slow.  Thus  we  find  the  Colorado  and  some  of  its  affluents 
flowing  in  the  deepest  and  most  magnificent  set  of  canyons  in  the 
world.  See  Fig.  34. 

Of  all  these  canyons  the  Grand  Canyon  in  Arizona  is  the  most  stupendous, 
and  one  of  the  most  impressive  wonders  of  the  world.  It  is  over  200  miles 
long  and  from  3000-5000  feet  deep  and  in  width  it  averages  about  10  miles. 
In  general  its  profile  shows  a  broader  upper  canyon  within  which  lies  a 


\j  v 

\   L 


Fig.  35.  —  Ideal  section  across  the  Grand  Canyon  (after  Button),     aa,  outer  canyon 
walls;  66,  inner  gorge;    1  and  3,  hard  resistant  beds;    2  and  4,  soft  beds. 


54 


TEXT-BOOK    OF    GEOLOGY 


deeper  inner  gorge,  as  illustrated  in  the  ideal  section  shown  in  Fig.  35.  It 
is  cut  in  horizontal  beds  of  rock  of  varying  degrees  of  hardness.  These  rest 
on  underlying  granite,  which  in  one  stretch  has  itself  been  cut  into  for  a  depth 
of  2000  feet  in  the  inner  gorge.  The  harder,  more  resistant  rock  layers  form 
cliffs  whose  talus  slopes  cover  the  softer  beds.  These  effects,  and  the  irregu- 
lar cutting,  carving  and  recessing  of  the  canyon  walls 
through  ravines  and  side  valleys,  have  given  rise  to 
enormous  and  striking  architectural  forms  and  ap- 
pearances, as  illustrated  in  Fig.  34.  Some  of  the 
masses  thus  carved  out  are  themselves  large  moun- 
tains. The  river  is  a  swift,  turbulent  stream,  turbid 
and  laden  with  silt,  from  200  to  300  feet  wide  and 
2400  feet  above  sea-level  opposite  Bright  Angel,  the 
place  in  Arizona  where  the  canyon  is  attained  at 
present  by  the  railroad,  and  thus  ordinarily  seen.  The 
Colorado  must  be  considered  as  a  young  river  in 
respect  to  the  character  of  its  valley,  and  having  in 
view  the  magnitude  of  the  task  which  it  has  yet  to 
accomplish  in  deepening  and  widening  the  valley.  In 
reality  it  was  a  rather  old  river,  but  it  has  been  re- 
juvenated, and  its  period  of  youth,  as  well  as  the 
work  it  has  to  do,  has  been  enormously  increased 
through  uprise  of  the  land,  a  matter  which  will  be 
considered  later. 

A  striking  instance  of  the  extreme  to  which  canyon 
of    cut,ting  may  go  is  seen  in  Fig.  36,  which  gives  a  section 
Qn  ^  head  waterg  Qf  ^  Rio  yi  one  of  the  trib. 

.         .  ,,      ~  ,        , 
utanes  of  the  Colorado. 


Fig.    36.       Section 
the  Rio  Virgen,  after 
G.  K.  Gilbert. 


Relation  to  Tributaries.  —  Examination  of  drainage  systems 
shows  that  in  a  vast  majority  of  cases  the  tributaries  of  a  river 
enter  it  at  grade,  at  the  same  elevation  as  the  main  stream.  They 
are  thus  said  to  be  accordant.  The  reason  for  this  is,  that  as  the 
main  stream  lowers  by  trenching,  the  resulting  increased  declivity 
which  is  given  the  tributaries  enables  them  to  keep  pace  in  spite  of 
the  smaller  volume  of  water.  But  this  may  increase  the  ratio  of 
the  trenching  of  the  lateral  valleys  over  their  widening  to  a  greater 
degree  than  in  the  main  trunk  valley  and  hence  they  may  be  pro- 
portionately narrower  and  steeper.  Instances  of  this  are  afforded 
by  some  of  the  tributaries  of  the  Colorado  River.  In  their  haste 
to  keep  accordant  relations  with  the  main  stream  they  have  cut 
very  narrow  canyons  whose  narrowest  profiles  are  almost  like  that 
shown  in  Fig.  36  ;  Kanab  Creek  is  an  example.  In  some  cases,  how- 
ever, in  the  younger  stages  of  normal  valleys,  small  streams,  unable 
to  keep  up  with  a  rapidly  downcutting  river,  are  obliged  to  cas- 
cade down  the  main  valley  walls. 

Waterfalls.  —  One  striking  feature  frequently  seen  in  young  val- 


RAIN    AND    RUNNING    WATER 


55 


leys  is  waterfalls,  the  result  of  the  unequal  erosion  of  rock  masses 
which  differ  in  resistance.  This  is  magnificently  illustrated  in  the 
great  cataract  at  Niagara,  which  may  thus  be  selected  as  an  ex- 
ample for  study.  The  Niagara  River,  which  drains  the  four  great 
upper  lakes,  in  its  course  of  36  miles  from  Lake  Erie  runs  over  a 
plateau  which  terminates  near  Lake  Ontario  in  an  escarpment  over 


Fig.  37.  —  General' view  of  the  Niagara  Falls. 

300  feet  high.  The  plateau  is  capped  by  a  hard  resistant  layer  of 
rock  known  as  the  Niagara  limestone,  under  which  are  soft,  easily 
eroded  Niagara  shales.  Originally  the  falls  was  situated  at  Lewis- 
ton  at  the  mouth  of  the  river,  and  falling  over  the  escarpment  had 
its  full  height  at  this  point.  These  relations  are  seen  in  Fig.  38. 
By  the  gradual  disintegration  and  undermining  of  the  softer  under- 
lying shale  the  harder  limestone  on  top  is  left  projecting  as  a  lip,  or 
table,  over  which  the  water  falls  as  shown  in  Fig.  39.  From  time  to 
time  the  projecting  table  rock,  left  unsupported  and  penetrated  by 
joint  cracks,  also  falls  and  is  carried  away.  By  means  of  this  ar- 
rangement, and  the  more  rapid  wear  of  the  underlying  beds,  the 
falls  maintains  itself  and  is  at  the  same  time  steadily  moving 
upstream,  leaving  a  deep  gorge  behind  it,  until  it  is  now  7  miles 
above  its  original  position. 

The  recession  of  Niagara  Falls,  and  the  rate  at  which  it  takes  place,  is  a 
matter  of  interest  and  has  been  the  subject  of  much  study  because  it  gives 


56 


TEXT-BOOK    OF    GEOLOGY 


an  idea  of  the  length  of  time  involved 
in  geologic  processes.  It  does  not 
seem  that  at  present  this  rate  can  be 
accurately  determined.  Some  hold  that 
the  main,  or  Canadian,  fall  is  receding 
at  a  rate  of  about  two  feet  a  year,  the 
American  fall  at  a  rate  much  less  than 
half  this;  others  say  more  than  twice 
as  much.  The  face  of  the  falls  is  now 
comparatively  broad,  about  4000  feet; 
when  it  was  contracted  in  the  narrow 
gorge  below,  its  width  was  about  one 
quarter  of  this  and,  owing  to  the 
greater  concentrated  weight  of  water, 
the  rate  of  recession  must  have  been 
more  rapid.  If  we  accept  an  average 
rate  of  five  feet  per  annum,  as  has 
been  assumed  by  some,  the  length  of 
time  involved  in  cutting  the  gorge  (7 
miles)  would  be  7000  years.  This  is  a 
minimum  estimate,  but  the  problem 
is  not  so  simple  as  this,  since  many 
factors,  involving  various  changes  in 
the  river  and  in  the  volume  of  its 
water  which  have  occurred  during  the 
past,  must  be  taken  into  account,  and 
some  estimates  which  have  done  this 
run  as  high  as  70,000  years.  Although 
we  do  not  know  the  length  of  time 
with  even  an  approach  to  accuracy 
these  estimates  are  of  value  in  that 
they  show  it  is  to  be  reckoned  in 
tens  of  thousands  of  years,  not  in 
hundreds,  nor  in  millions.  The  height 
of  the  falls,  which  is  now  about  160 
feet,  diminishes  as  they  move  because 
the  layer  of  Niagara  limestone  which 
conditions  them  dips  gently  downward 
upstream. 

Many  other  famous  waterfalls  are  due  to  an  arrangement  of  rocks  similar 
to  that  at  Niagara,  such  as  the  falls  of  St.  Anthony  on  the  upper  Mississippi 
at  Minneapolis,  and  its  tributary  streams,  which  fall  into  the  gorge  below; 
the  Shoshone  Falls  on  the  Snake  River  in  Idaho;  those  on  the  tributaries  of 
the  Columbia  River,  and  some  smaller  falls  in  New  York  State  like  Trenton 
PaUs. 

But  falls  may  be  produced  in  other  ways  as  well,  by  glaciers,  as  will  be 
noticed  later,  by  the  accidental  damming  of  streams  by  lava  flows  or  land- 
slides, and,  as  Dana  has  shown,  they  are  a  natural  result  of  the  mature  erosion 
of  the  headwaters  of  streams  in  mountain  regions  whose  declivities  become 
steepened  by  erosive  processes. 


Fig.  38.  —  Map  of  Niagara  Falls  and 
River,  after  G.  K.  Gilbert. 


RAIN    AND    RUNNING    WATER 


It  is  clear  that  waterfalls, 
whether  occasioned  like  Niagara 
by  unequal  hardness  of  rocks,  or 
due  to  obstructions  in  the  course 
of  a  stream,  or  to  some  previous 
geological  action,  cannot  indefi- 
nitely persist ;  they  must  be  worn 
away  in  time  and  disappear,  for 
reasons  previously  stated  under 
river  grading.  The  more  sedi- 
ment a  river  carries,  the  less 
likely  falls  are  to  be  found  in 
its  course,  or  the  shorter  will 
be  their  life.  Thus  they  are 


Fig.  39.  —  Section  showing  rock  layers  and 
cause  of  falls  at  Niagara.  (After  G.  K. 
Gilbert.)  N.  L.,  Niagara  limestone  with 
soft  shale  below;  C.  L.,  Clinton  limestone 
with  shales  and  sandstones  below.  300  ft. 
=  1  inch.  W.  L.  =  water  level  of  pool. 


commonly  regarded  as  signs  of  topographic  youth  in  regions  where 
they  occur. 

Pot-Holes.  —  A  minor  feature,  seen  in  the  bed  of  rapid,  swirling 
streams,  consists  in  the  presence  of  pot-holes,  Fig.  40.  These  are 
circular  excavations  worn  in  bed-rock  by  the  whirling  action  of 
eddies.  If  the  conformation  of  the  stream  bed  is  such  that  an  eddy 
persists  in  one  place,  the  water  whirls  sand  and  gravel  with  it,  and 
this  bores  downward;  although  the  material  wears  out  in  grinding, 
it  is  continually  replaced  by  fresh,  and  the  process  continues.  Such 
pot-holes  may  have  diameters  from  a  few  inches  up  to  50  feet,  and 
the  depth  may  vary  to  a  similar  extent,  or  be  even  greater.  They 
are  of  interest  in  that  they  indicate  so  clearly  the  action  of  whirling 
water  and,  occurring  not  infrequently  in  country  rock  now  far  from 
any  stream,  they  prove  that  at  one  time  it  was  the  bed  of  a  rapid 
current. 


Constructive  Work  of  Rivers 

So  far  in  the  study  of  rivers  we  have  considered  the  destructive, 
erosional  work  which  they  perform  —  work  done  chiefly  in  their 
upper  reaches  and  seen  in  the  valleys  they  excavate  in  the  higher 
lands.  Some  rivers  have  a  swift  course  through  elevated  tracts  of 
country  to  the  sea,  their  work  is  cut  short  when  they  enter  it,  and 
they  deliver  their  burden  at  once;  but  many,  and  especially  the 
largest  rivers  of  the  world,  descend  into  wide  lowlands,  through 
which  with  steady  current  they  wind  to  their  journey's  end.  In  these 
lowlands,  and  at  the  river's  mouth,  the  work  done  is  different  from 
that  in  the  upper  reaches ;  it  is  largely  constructive,  rather  than  de- 


58 


TEXT-BOOK    OF    GEOLOGY 


structive,   and   consists  mainly   in  the  deposition  of  the   burden 
assumed  through  erosion  in  the  higher  part  of  the  course. 


Fig.  40.  —  Pot-holes  worn  in  granite  rock  by  stream  action.     Tuolumne  River,  Cal. 

U.  S.  Geol.  Surv. 

Flood-Plains.  —  The  lowlands  situated  on  the  lower  courses  of 
rivers  are  subject  annually  to  floods  caused  by  spring  rains  and  the 
melting  of  snows  in  the  mountains.  Unless  otherwise  checked  the 
river  overflows  its  banks  and  spreads  widely  a  vast  volume  of 
muddy  water  over  these  flat  lands.  The  country  may  appear  as  a 
great  lake  for  many  miles  outward  from  the  course  of  the  river.  As 
the  velocity  of  the  water,  except  in  the  main  channel,  is  checked  in 
spreading  outward  it  deposits  its  burden  of  fine  mud  and  silt. 
Finally  the  waters  recede  leaving  the  deposit  of  mud  behind.  Such 
a  deposit  is  known  as  alluvium  and  the  flat  lands  along  the  lower 
courses  of  rivers,  built  up  by  these  successive  deposits,  are  often 
called  alluvial  plains.  Rivers  running  through  their  alluvial  plains, 
like  the  Mississippi,  are  generally  in  that  phase  of  development  or 
stage  of  their  history,  which  has  been  previously  explained  as 
graded.  The  condition  of  the  country  through  which  they  are 
passing  will  be  considered  later  under  the  heading  of  baselevel. 
For  convenience  in  description  the  whole  flood  plain  may  be  di- 
vided into  two  parts,  the  river  flats  or  swamps,  and  the  delta. 

River  Flats  and  Swamps.  —  Examination  of  the  alluvial  plains 


RAIN    AND    RUNNING    WATER 


59 


along  the  lower  courses  of  rivers  shows  that  in  general  the  land  is 
somewhat  higher  next  to  the  river  and  slopes  away  as  one  goes  from 
it.  The  reason  of  this  is,  that  in  times  of  flood,  when  the  river  over- 
flows its  banks,  the  overflowing  muddy  water,  having  its  velocity 
checked  as  it  leaves  the  main  current  of  the  river,  at  once  com- 
mences to  deposit,  and  the  deposits  are  therefore  most  abundant  near 
the  stream,  and  composed  of  the  coarsest  material  carried  by  it. 
The  low  ridges  formed  in  this  way  are  often  called  natural  levees. 
Beyond  these  the  land  is  low,  more  or  less  ill  drained,  and,  in  humid 
regions,  commonly  covered  with  trees  and  other  vegetation  and 
thus  of  the  nature  of  swamps.  The  river  plain  of  the  Mississippi  is 
estimated  to  cover  an  area  of  30,000  square  miles,  and  a  large  por- 
tion of  it  consists  of  extensive  swamps. 


Fig.  41.  —  Illustrates  bars  and  deposits  made  by  an  aggrading  stream  in  a  flat  part 
of  its  course.  Junction  of  Cooper's  River  and  the  Yukon.  W.  C.  Mendenhall, 
U.  S.  Geol.  Surv. 

Although  river  flats  and  swamps  are  most  natural  and  prominent  in  the 
lower  reaches  of  streams,  they  may  occur  in  any  part  of  its  course  where  a 
sudden  lessening  of  its  grade  may  cause  it  to  deposit  extensively,  or  aggrade, 
see  Fig.  41.  The  stream  would  here  build  up  a  flat,  gently  inclined  area 
through  which  it  would  wind  with  a  steady  current;  beyond  this  it  would 
again  descend  and  regain  its  erosive  power.  Thus,  while  such  flats  are  built 
up  at  the  upper  end,  they  are  being  carried  away  by  rain  and  river  work  at 
the  lower  one.  Ultimately,  as  the  river  becomes  graded,  they  must  dis- 
appear and  hence  they  are  temporary  lodgments  of  material,  as  contrasted 
with  the  final  river-plain  which  must  endure  as  long  as  the  land  is  affected 
by  the  same  set  of  geological  conditions.  In  many  cases  these  upper  river 


60 


TEXT-BOOK    OF   GEOLOGY 


flats  represent  lakes,  or  ponds,  through  which  the  stream  passed  and  which 
have  been  filled  up  and  obliterated  by  deposit.  The  same  characters  and 
river  work,  which  are  features  of  the  great  alluvial  plains,  may  be  seen  in 
them  on  a  smaller  scale. 

Deltas:  Mode  of  Formation.  —  The  lower  river  plain  is  fre- 
quently continued  by  an  area  of  similar  flat,  low-lying  land  extend- 
ing into  the  sea  or  lake  into  which  the  stream  discharges.  This 
tract  usually  has  a  triangular  shape  with  one  apex  pointing  up- 
stream, from  which  fact  it  has  received  the  name  of  delta,  since  it 


MEDITERRANEAN          SEA 

Rosetta  Mouth       x^S?  ^-^^  Damietta  Mouth 


Hosett 
Abouki 


rtSaid 


Fig.  42.  —  Delta  of  the  Nile,  showing  its  form  and  distributaries. 

has  a  shape  similar  to  the  Greek  letter  so  called.  On  reaching  it  the 
river  usually  splits  into  many  branching  streams  which  wander 
through  the  delta  in  a  roughly  fan-shaped  arrangement  as  illus- 
trated in  Fig.  42.  These  branches  are  called  distributaries.  The 
delta  represents  land  which  has  been  formed  by  the  river  and  re- 
claimed from  the  sea  or  lake.  Although  a  great  part  of  the  burden 
of  sediment  carried  by  the  river  may  be  deposited,  as  we  have  seen, 
upon  its  alluvial  plain,  another  large  portion  is  carried  on  to  the 
river's  mouth.  On  meeting  still  water,  the  current  is  checked  and  the 
sediment  in  suspension  is  deposited,  while  that  moved  along  the 
bottom  comes  to  rest.  Through  this  continued  deposition  land  is 
formed,  and  the  growing  land  pushes  seaward.  As  the  current  of 
the  river  urges  forward  some  distance  into  the  body  of  water  into 


RAIN    AND    RUNNING    WATER  61 

which  it  discharges,  deposit  takes  place  along  the  sides  of  this  cur- 
rent, as  well  as  at  the  point  where  it  ceases,  and  this  forms  seaward 
a  continuation  of  the  natural  levees.  At  the  place  where  the  cur- 
rent stops,  the  heavier  material  is  promptly  dropped  and  this  builds 
up  a  bar,  or  bars,  across  the  mouth  of  the  river.  The  rising  bars 
may  finally  obstruct  the  course  of  the  river  to  such  an  extent  that 
it  breaks  through  the  natural  levees  at  some  point  upstream  and 
seeks  an  outlet  elsewhere,  leaving  a  diminished  volume  of  water 
escaping  by  the  old  channel.  The  new  outlet  goes  through  the  same 
process  and  thus,  by  upbuilding  natural  banks  and  bars  and  by 
breaking  through  them,  the  branching  system  of  distributaries  is 


Fig.  43.  —  Delta  of  the  Yahtse  River,  Alaska.     I.  C.  Russell,  U.  S.  Geol.  Surv. 

formed  and  shape  given  to  the  delta.  The  branching  system  ex- 
tending seaward,  as  illustrated  in  Fig.  43,  represents  the  skeleton 
of  the  growing  delta;  between  these  arms  lie  very  shallow  basins 
which  gradually  fill  up  with  finer  material  and  thus  become  land, 
at  first  mud  flats  or  swamps,  and  then  more  solid  land  as  the  an- 
nual overflows  build  it  up  by  their  deposits.  The  result  is  seen 
in  Fig.  44. 

The  structure  of  deltas,  as  produced  by  the  deposits  under  differ- 
ent conditions,  will  be  considered  in  a  later  chapter. 

Conditions  Necessary  for  Deltas:  Examples.  —  Deltas,  espe- 
cially those  of  great  rivers,  are  commonly  formed  of  very  fine  muds 
and  silts;  as  the  river  descends  into  its  plain,  its  velocity  is  di- 
minished, the  heavier  and  coarser  material  is  dropped,  the  gradient 
is  further  lessened  by  winding,  and  strength  to  carry  only  the  finer 


62 


TEXT-BOOK  OF  GEOLOGY 


IN  1852 

SCALE  OF  MILES 


IN  1905 


Fig.  44.  —  Illustrating  the  growth  of  the  Mississippi  delta  during  50  years,  after 

G.  R.  Putnam. 


RAIN  AND  RUNNING  WATER  63 

sediments  remains.  It  now  becomes  a  question  between  the  amount 
and  coarseness  of  sediment  which  the  river  can  discharge,  and  the 
ability  of  the  waves  and  currents  in  the  body  of  water  into  which  it 
enters,  owing  to  their  extent  and  strength,  to  sweep  it  away  and 
prevent  delta  formation.  Hence  rivers  entering  lakes  and  enclosed 
seas,  such  as  the  Gulf  of  Mexico,  the  Black,  Caspian,  and  Mediter- 
ranean seas,  where  waves  and  tidal  currents  are  weak,  form  deltas. 
This  is  illustrated  by  the  Mississippi,  the  Danube,  the  Volga  and 
the  Nile,  all  of  which  have  deltas.  It  may  also  happen  that,  even 
in  seas  where  there  is  considerable  tide  and  rather  strong  currents, 
the  conditions  may  be  such,  and  the  volume  of  sediment  so  large, 
that  rivers  can  form  deltas.  This  is  illustrated  by  the  Rhine,  the 
Niger,  the  Ganges  and  the  Hoangho  rivers.  The  Thames  on  the 
other  hand  forms  no  delta  because  the  strong  tidal  currents  along 
the  English  coast  sweep  the  sediments  away.  On  the  Atlantic  coast 
of  North  America  there  are  no  deltas  because  this  portion  of  the 
continent  in  a  very  recent  geological  period  has  undergone  consid- 
erable subsidence,  such  deltas  as  the  rivers  may  have  previously 
formed  have  been  submerged,  the  ocean  has  entered  the  river  val- 
leys, flooding  them  and  converting  them  into  bays  and  estuaries,  and 
the  rivers  are  now  depositing  at  the  heads  of  these  estuaries  seeking 
to  fill  them  up  and  form  new  deltas.  If  the  floor  of  the  ocean  is 
sinking,  as  appears  to  be  commonly  the  case  under  very  large 
deltas,  it  then  becomes  a  question,  between  the  rate  of  upbuilding 
by  river  deposit  and  that  of  the  subsidence,  whether  the  river  can 
succeed  in  maintaining  a  land  area  of  its  delta,  or  not.  It  must  not 
be  forgotten  that  in  any  case  a  considerable  portion  of  the  delta 
deposit  on  its  seaward  slope  is  under  water.  This  extension  con- 
sists of  the  finest  material  which  is  carried  out  some  distance  before 
being  deposited,  and  it  constitutes  the  submarine  platform  on  which 
the  landward  area  rises. 

The  deltas  of  great  rivers  form  large  areas  of  land.  That  of  the  Nile  is  nearly 
100  miles  long  and  200  broad  on  its  seaward  front;  that  of  the  Ganges  (and 
Brahmaputra)  200  miles  long  and  its  area  possibly  40,000  square  miles.  The 
Mississippi  delta  is  200  miles  long,  its  area  over  12,000  square  miles,  and  the 
thickness  of  the  deposit  over  800  feet.  The  great  thickness  is  due  to  com- 
bined subsidence  of  the  sea-floor  and  deposit  by  the  river. 

The  ratio  of  growth  of  deltas  depends  on  a  variety  of  circumstances.  The 
Mississippi  has  been  estimated  to  be  pushing  forward  into  the  Gulf  at  a  rate 
of  a  mile  in  16  years.  This  rate  is  probably  more  rapid  than  in  former  times 
for  several  reasons.  The  country  drained  by  the  river  and  its  tributaries  is 
now  widely  settled  and  cultivated,  and  with  occupancy  of  the  land  has  come 
extensive  destruction  of  forests  and  the  upturning  and  exposure  of  the  soil 


64  TEXT-BOOK    OF   GEOLOGY 

for  agriculture.  This  produces  a  more  rapid  erosion  of  the  basin  and  a  larger 
volume  of  sediment.  The  flood  plain  is  now  mostly  protected  from  the 
annual  overflows  by  a  great  system  of  levees,  and  this  has  transferred  the 
sediment  which  would  otherwise  be  deposited  upon  it  to  the  extension  of 
the  delta. 

Artificial  Levees.  —  The  control  and  management  of  large  rivers  in  their 
flood  plains  is  one  of  the  most  serious  problems  in  engineering  that  civilization 
has  to  deal  with.  Alluvial  plains  and  deltas  are  composed  of  fine,  rich  and 
fertile  soil,  and  hence  are  apt  to  be  much  cultivated  and  thickly  populated. 
These  lowlands  are  protected  from  the  annual  overflow  by  levees,  as  men- 
tioned above,  which  raise  the  natural  ones  above  the-  level  of  high  water.  In 
the  Mississippi  the  increased  volume  of  confined  water,  by  adding  to  the 
strength  of  the  current,  has  increased  the  scour  and  deepened  the  channel. 
The  case  of  the  Po  and  other  rivers  long  leveed  would  appear  to  indicate  that 
this  can  only  be  temporary,  though,  owing  to  the  fineness  of  sediments,  it 
may  long  continue.  With  increased  current  coarser  material  must  be  de- 
posited farther  and  farther  downstream,  and  the  river  bed  must  gradually 
rise,  and,  correspondingly,  the  levees  must  also  be  raised.  This  has  been 
done  on  the  lower  Po  to  such  an  extent  that  the  river  bed  and  confining 
levees  are  stated  to  be  above  the  tops  of  the  houses  in  places  on  the  lower 
plain.  The  accidental  breaking  of  the  levees  entails  wide  flooding  of  the 
river  plain  and  great  disaster.  This  has  occurred  a  number  of  times  on  the 
Mississippi,  and  notably  on  the  Hoangho  River  in  China,  entailing  in  the  lat- 
ter case  enormous  loss  of  life. 

The  obstructions  to  navigation  caused  by  the  bars  at  the  mouths  of  great 
rivers  have,  in  the  case  of  the  Mississippi,  been  successfully  removed  by  the 
building  of  crib-works,  called  jetties,  in  such  a  way  as  to  prolong  the  natural 
levees  out  in  shallow  water,  and  to  thus  continue  the  current  of  the  stream, 
so  that  the  load  of  sediment  is  deposited  in  deep  water  instead  of  on  the 
shallow  submarine  top  of  the  delta,  while  the  increased  scour  deepens  the 
channel. 

Alluvial  Cones  or  Fans.  —  When  a  swift  tributary  stream  enters 
into  the  wider  and  more  level  valley  of  a  larger  river  the  sudden 
change  in  gradient  may  cause  it  to  deposit  the  greater  part  of  its 
burden  on  the  floor  of  this  valley.  In  this  way  semi-cones,  or  fan- 
shaped  elevations,  of  deposits  are  formed.  They  may  be  regarded 
as  deltas  formed  on  land,  but  differ  from  the  true  deltas  made  in 
water  by  their  shape  and,  generally,  by  the  coarser  material  com- 
posing them.  A  view  of  such  an  alluvial  cone  is  seen  in  Fig.  45. 
They  are  often  conspicuous  in  the  broader  valleys  in  arid  districts 
where  material  is  washed  out  of  the  narrower  tributary  ravines  by 
sudden  heavy  downpours  of  rain  and  deposited.  In  these  regions 
whole  basins  may  be  filled  by  such  rain  wash  deposits  and  present 
very  level  floors. 

Meanders.  —  A  river,  either  in  its  upper  flats  or  in  its  ultimate 
flood-plain,  has  a  relatively  gentle  flow  owing  to  the  low  gradient, 
and  in  consequence  diminished  strength  of  current.  It  is  therefore 


RAIN  AND  RUNNING  WATER 


65 


Fig.  45.  —  Alluvial  cone,  made  by  a  tributary  to  a  larger  stream.     Stoughton,  Wis. 
W.  C.  Alden,  U.  S..  Geol.  Surv. 


Fig.  46.  —  Meanders  of  a  stream  in  a  nearly  flat  region.       Trout  creek,  Yellowstone 
Park.     C.  D.  Walcott,  U.  S.  Geol.  Surv. 


66 


TEXT-BOOK    OF    GEOLOGY 


easily  turned  by  obstacles,  and  tends  to  wander  in  the  plain  in  a 
series  of  winding  curves  called  meanders.  Nor  does  it  long  main- 
tain a  set  course  but  shifts  about  by  changing  its  course  and  forming 
new  curves.  Even  if  artificially  straightened  it  would  soon  begin 
to  wander.  The  reason  for  this  is  as  follows:  The  swiftest  part  of 
a  river  current  is  normally  in  the  middle  of  the  stream;  if  it  en- 
counter any  obstacle,  such  as  a  rock  or  stump,  it  is  deflected  against 
the  opposite  bank.  The  latter  will  be  eroded,  and  this  will  tend  to 
throw  the  current  against  the  opposite  bank  (as  in  A,  Figs.  47  and 
48)  which  will  in  its  turn  be  eroded.  Thus  meanders  begin  to  form 


Fig.  47.  —  Section  of  a  stream  channel  from       Fig.  48.  —  Illustrating  the  formation  of 
A  to  B  shows  it  to  have  the  profile  seen  meanders  and  ox-bows, 

above,  the  deepest  part  lying  close  in  to 
the  bank  at  A.  See  Fig.  48,  A  and  B 
the  same. 

and  a  continuation  of  the  process  increases  them.  Meanwhile  the 
current  being  slackened  at  B,  Fig.  48,  deposit  takes  place  there  and 
the  point  grows  out  as  the  hollow  recedes.  Further  stages  of  the 
process  are  shown  in  C  and  D.  Eventually  in  the  process  of 
meandering,  a  loop,  as  at  E,  is  cut  through,  leaving  an  island  in  the 
river,  the  main  current  takes  the  shortest  route  FF,  and  the  en- 
trances to  the  abandoned  channel  HJ  become  silted  up,  leaving  a 
shallow  crescentic  lake.  Such  lakes  are  called  ox- bows ;  they  are  com- 
mon along  the  Mississippi  and  other  rivers  on  their  alluvial  plains ; 

they  gradually  fill  up  and  be- 
come sloughs  or  marshes,  and 
finally  perhaps  low  meadows, 
though  retaining  the  original 
form  by  which  their  former 
character  may  be  recognized. 
Such  old  abandoned  channels 


Fig.  49.  —  A  river  widening  its  valley  by 
meandering  and  planation. 


are    very    common   on    river 
flats  and  plains. 

Work    on    River    Plain; 

Planation.  —  If  a  river  is  graded  either  temporarily  upon  an 
upper  flat,  or  on  its  plain,  it  neither  lowers  its  channel  by  cutting 
down  (corrasion)  nor  builds  it  up  by  deposit  (aggrading).  In  this 
case,  as  it  wanders  by  meandering  at  the  same  level  from  side  to  side 


RAIN  AND  RUNNING  WATER  67 

in  its  valley,  it  impinges  against  the  valley  bluffs  from  time  to  time, 
as  at  A  in  Fig.  49,  cuts  them  down  by  undermining,  and  carries 
the  material  away.  By  continuation  of  this  process  the  valley  is 
widened,  and  this  work  is  known  as  lateral  planation. 

There  appears  to  be  a  certain  relation  between  the  width  of  a  river  and 
that  of  the  belt  within  which  it  can  meander.  A  recent  estimate  places  the 
width  of  the  latter  at  18  times  that  of  the  average  width  of  the  river.  It 
will  be  noticed  that  in  meandering  a  river  lessens  its  gradient,  and  this  is  a 
way  in  which  it  seeks  to  maintain  itself  at  grade.  The  patterns  produced  by 
many  rivers,  wandering  on  their  alluvial  plains,  are  of  wonderful  intricacy. 

Terraces.  —  There  frequently  occur  in  river  valleys  long  narrow 
stretches  of  very  flat  and  nearly  level  land,  often  on  both  sides  of 
the  river.  Back  of  them  on  the  side  away  from  the  stream  may 


Fig.  50.  —  Terraces  on  the  Fraser  River,  opposite  Lillooet,  British  Columbia. 
A.  M.  Bateman,  Geol.  Surv.  of  Canada. 

rise  the  ascending  slopes  of  the  valley,  next  to  the  river  they  may 
descend  to  the  present  river  plain  steeply,  or  even  in  bold  bluffs. 
Often  there  are  several  of  them  raised  one  above  the  other  like  steps. 
or  shelves.  These  are  known  as  terraces;  they  are  composed  of 
alluvium,  of  river-deposited  sands,  gravels,  etc.,  and  they  are  the 
remnants  of  former  river-plains,  when  the  beds  of  streams  were 
at  their  altitude.  They  are  illustrated  in  Fig.  50.  They  may  be 
formed  in  a  variety  of  ways,  one  of  the  most  important  of  which  is 
as  follows:  A  stream  may  at  one  time  in  its  earlier  history  be 
heavily  burdened  with  sediment  and  deposit  this  at  points  of 
lessened  gradient,  forming  river  flats  and  an  ultimate  flood-plain. 


68  TEXT-BOOK   OF   GEOLOGY 

At  a  later  time,  with  decreased  load  and  therefore  renewed  energy, 
it  may  begin  eroding,  cut  into  these  deposits,  start  a  new  flood-plain, 
or  flat,  at  a  lower  level,  and  thus  leave  the  remnants  of  the  former 
plain,  or  flat,  as  terraces,  as  shown  in  Fig.  51.  As  the  river  swings 
from  side  to  side  in  -this  work  of  planation  and  removal  it  is  apt  to 
expose  resistant  masses,  such  as  rock  ledges,  buried  in  the  old  de- 
posits. These  may  turn  the  course  of  the  stream,  and  defend  a 
portion  of  the  former  flood-plain  from  being  carried  away.  They 
usually  mark  the  situation  of  projecting  headlands  in  the  terrace 
bluff,  which  recedes  from  them  on  either  side,  as  may  be  seen  in 
Fig.  49.  Changes  in  the  amount  of  water  discharged  in  different 


Fig.  51.  —  Illustrating  formation  of  terraces.  A  A,  Section  of  river-cut  valley;  B,  al- 
luvial deposits  of  river;  tt,  former  flood-plain,  now  forming  terraces;  c,  new  flood 
plain. 

periods  might  cause  a  similar  result.  On  an  upper  river  flat  the 
increased  gradient  at  the  lower  end,  normally  working  upstream 
and  deepening  the  channel,  may  leave  remains  of  the  former  flat 
as  terraces.  Other  agencies  contributing  to  their  formation  will  be 
described  later.  Owing  to  their  low  grades  and  situations  in  river 
valleys  they  have  been  extensively  used  in  the  location  of  railway 
lines. 

Structure  of  River  Deposits;  Stratification.  —  Before  leaving 
the  constructive  geological  work  done  by  rivers,  it  should  be  stated 
that  all  deposits  by  them,  as  indeed  by  all  currents  of  water,  whether 
rivers  on  the  land  or  tidal  currents  in  the  sea,  are  so  laid  down, 
through  the  sorting  activity  of  water,  that  they  consist  of  distinct 
layers,  or  beds,  of  varying  degrees  of  fineness.  Usually  these  beds 
are  very  regularly  parallel  for  greater  or  lesser  distances,  and  de- 
posits which  exhibit  this  laminated,  banded,  or  bedded  appearance 
are  said  to  be  stratifi-ed,  and  the  arrangement  is  called  stratification. 
Whether  on  river  flats,  on  alluvial  plains,  or  in  the  delta,  river  de- 
posits are  thus  stratified.  This  subject,  and  the  importance  of  its 
bearing  in  understanding  the  origin  of  a  great  class  of  rocks,  will  be 
discussed  in  detail  later,  under  the  structural  side  of  Geology. 


RAIN  AND  RUNNING  WATER  69 

Some  Phases  of  River  History 

Study  of  the  land  surfaces  of  the  earth  has  shown  us  that  they 
are  not  permanent  geological  features.    The  expression  "the  ever- 
lasting hills"  often  used  in  literature  has  value  only  in  reference  to 
the  duration  of  human  life ;  geologically  it  has  no  significance.    For 
not  only  are  the  lands  subject  to  change  through  erosion,  as  we  have 
seen  in  the  preceding  pages,  but  they  suffer  changes  of  level,  with 
reference  to  the  sea,  through  raising  or  sinking  of  the  earth's  crust. 
This  has  often  occurred  in  the  past,  and  to  such  an  extent  that  sea- 
bottoms  have  become  land,  or  land  surfaces  have  sunk  to  become 
sea-bottoms.     Not  only  has  this  happened  in  the  past,  but  it  is 
Burring  now,  though  so  slowly  that  only  one  trained  in  geology  is 
ible  to  perceive  it.    The  land  surfaces  also  have  been  profoundly 
lodified  at  times  by  warpings,  by  being  covered  with  huge  areas  of 
},  and  in  other  ways.    We  need  not  stop  to  treat  these  matters 
lere,  since  we  shall  study  them  in  detail  in  appropriate  places,  ex- 
pt  to  understand  that  great  changes  of  land  have  occurred,  and 
consider  their  effect  upon  the  life  history  of  rivers. 

Since,  therefore,  new  lands  have  appeared  from  time  to  time,  or 
)ld  ones  have  had  new  levels  and  surfaces  given  them,  it  is  evident 
tat  new  drainages  have  also  been  initiated.  For  a  long  time,  such 
drainages  have  the  characteristics  of  topographic  youth;  the  upper 
valleys  are  narrow  and  V-shaped,  the  ridges  between  are  wide  and 
often  flat;  there  may  be  lakes  not  yet  filled  up,  waterfalls  not  yet 
eroded  away,  and  similar  features.  As  time  goes  on,  these  are  ob- 
literated, the  land  is  everywhere  carved  into  drainage  slopes,  the 
main  valleys  are  widened,  the  hill  slopes  become  gentle  and  rounded, 
the  rivers  meander  on  their  valley  floors,  they  become  graded,  with 
harmonious  relation  between  erosion  and  transportation;  in  other 
words  they  are  mature.  Only,  as  previously  stated  on  page  5J,  these 
are  relative  terms,  not  ones  of  absolute  time.  We  may  now  con- 
sider further  stages  of  river  history  and  land  erosion,  or,  as  the 
latter  is  sometimes  called,  degradation. 

Baselevel.  —  If  that  work  of  subaerial  erosion,  performed  by 
weathering  and  by  rain  and  running  water,  were  to  continue  un- 
checked upon  a  land  area,  its  surface  would  be  gradually  lowered, 
and,  in  measure  as  the  process  continues,  the  material  taken  away 
by  the  carrying  streams  is  deposited  in  the  sea.  But  the  work  of 
the  streams  when  they  reach  the  ocean  is  finished;  they  can  do  no 
more  eroding,  and  their  burdens  must  be  laid  down.  The  level  of 
the  sea  therefore  is  that  below  which  land  cannot  be  eroded  by  the 


70  TEXT-BOOK    OF   GEOLOGY 

agents  of  subaerial  erosion  mentioned  above.  We  may  therefore 
conceive  of  the  sea-level  as  an  imaginary  surface  extended  under 
the  land,  which  represents  the  limit  to  which  stream  erosion  and 
weathering  seek  to  bring  downward  the  land  areas.  It  is  the  ulti- 
mate baselevel  of  erosion  by  these  factors,  or,  as  stated  by  Davis, 
"the  limit  of  subaerial  erosion  is  the  'level  base'  or  'baseleveF  drawn 
through  a  land  mass  in  prolongation  of  the  normal  sea  level  surface." 
If  we  conceive  of  erosion  continued  indefinitely,  in  its  final  stage 
a  land  area  would  actually  be  reduced  almost  to  sea-level,  but  not 
quite;  it  would  be  nearly  flat  with  a  gradient  so  low  that  it  would 
be  just  sufficient  to  shed  the  rainfall  seaward;  erosion  and  deposi- 
tion would  have  therefore  ceased ;  the  drainage  would  have  no  par- 
ticular channels  and  would  pass  off  as  from  a  very  flat  roof.  This 
is  as  far  as  erosion  could  go ;  the  land  would  have  been  very  nearly 
baselevelled,  but  not  entirely  so.  Baselevel  then  is  the  imagined 
plane  (extended  sea-level)  "towards  which  the  land  surface  con- 
stantly approaches  in  accordance  with  the  laws  of  degradation,  but 
which  it  can  never  reach."  (Davis.) 

It  is  evident  from  what  has  been  said  above,  and  from  foregoing  pages, 
that  no  land  has  ever  been  absolutely  baselevelled;  enough  of  a  slope  must 
remain  to  carry  off  the  drainage.  By  sea-level  is  here  meant  the  normal 
average  level  of  the  sea  without  regard  to  the  tides,  or  to  those  changes 
which,  as  we  shall  see  later,  have  affected  the  ocean  levels  in  times  past.  The 
fact  that  rivers,  like  the  Mississippi,  for  example,  may  erode  their  channels, 
as  they  enter  the  sea,  as  much  as  250  feet  below  sea-level,  does  not  invalidate 
the  mathematical  conception  of  baselevel  described  above.  Upper  flats  and 
plains,  upon  which  streams  meander  and  through  whose  extent  they  are 
graded,  as  well  as  lakes  into  which  they  may  empty,  are  often  spoken  of  as 
temporary  or  local  baselevels. 

Peneplain.  —  Since  the  work  of  erosion  progresses  more  and  more 
slowly  as  the  heights  and  gradients  decrease,  it  is  evident  that  the 
length  of  time  required  to  bring  a  land  down  to  a  plain  almost  at 
sea-level,  as  described  above,  would  be  enormously,  almost  in- 
definitely, lengthened  out  toward  the  latter  end  of  the  process. 
Long  before  this  could  happen  the  land  surface,  especially 
the  portion  marginal  to  the  sea,  would  be  reduced  to  a  low  country 
of  small  variation  in  its  general  relief.  Its  elevations  would  be 
broad  and  rolling,  with  very  gentle  slopes,  and  would  rise  every- 
where to  the  same  general  height ;  between  them  would  be  wide  and 
very  shallow  basins,  in  which  the  streams  would  wander  more  or 
less  sluggishly  to  the  sea,  in  a  well-graded  condition.  There  would 
be  no  waterfalls  and  no  lakes.  As  one  gazes  over  such  a  landscape 


RAIN    AND    RUNNING    WATER 


71 


he  sees  its  elevations  merge  into  one  level  line  at  the  horizon.  In 
addition,  whatever  may  be  the  nature  and  structure  of  the  under- 
lying country  rocks,  whether  hard  or  soft,  these  features  would 
everywhere  persist.  A  land  surface  in  this  stage  of  wearing  down 
(degradation)  by  erosion  is  known  as  a  peneplain  (almost  a  plain), 
see  Fig.  54,  and  its  formation  is  often  referred  to  as  peneplanation. 
A  peneplain  then,  is  a  very  old  erosional  land  surface,  of  low  or  faint 
relief,  sheeted  over  with  a  graded  soil  cover. 

The  vast  plain  of  central  Russia  has  been  cited  as  a  good  example  of  a 
modern  peneplain;  it  has  been  slightly  raised  and  the  rivers  have  been 
set  at  work  again  eroding.  Ancient  peneplains,  which  have  been  uplifted  and 


Fig.  52.  —  Drowned  river  mouth  forming  an  estuary.     Balaklava,  Crimea,  Russia. 

then  carved  by  the  streams  into  tracts  of  hilly  country,  have  been  recog- 
nized in  many  places,  such  as  southern  New  England,  Pennsylvania,  central 
Missouri,  and  the  south  of  England. 

It  may  happen  that  here  and  there  isolated  hills  may  rise  above  the  gen- 
eral surface  of  a  peneplain,  like  islands  above  a  sheet  of  water.  They  are 
composed  of  harder  or  more  resistant  rocks  or  masses  so  situated  at  the 
heading  of  streams  as  to  be  the  last  residuals,  which  erosion  has  not  yet 
been  able  to  reduce  to  the  general  level.  Such  island  masses  have  been  called 
monadnocks,  after  Mt.  Monadnock,  which  rises  above  the  New  England 
peneplain. 

It  should  be  noted  that  topographic  forms  somewhat  similar  to  peneplains 
may  be  made  by  long  continued  erosion  of  the  sea  or  by  the  work  of  the 
ice  glaciers,  as  will  appear  later;  they  may  appear  also  in  arid  regions. 


72 


TEXT-BOOK    OF   GEOLOGY 


Drowned  and  Revived  Rivers.  —  If  a  land  surface  should  be 
lowered  sufficiently  by  subsidence  the  lower  valleys  of  rivers  would 
be  flooded  by  the  sea  and  would  become  estuaries,  like  Delaware 
and  Chesapeake  bays.  In  this  case  the  river  is  said  to  be  drowned. 
See  Fig.  52.  The  tributary  rivers  which  formerly  ran  into  the  main 
stream  are  called  dismembered.  These  relations  are  illustrated  in 
Fig.  53.  If  a  river  were  carried  below  baselevel  by  drowning,  it 
would  begin  to  fill  up  the  estuary  by  depositing,  and  this  work  would 

go  on  until  the  whole  bay  is  filled  up 
and  converted  into  a  river-plain,  which 
in  a  sense  may  be  regarded  as  its  delta. 
This  is  the  condition  of  the  rivers  and 
estuaries  along  the  Atlantic  coast;  in 
which  the  remaking  of  the  drowned  river- 
plain  is  only  partly  completed,  as  shown 
in  Fig.  53. 

On  the  other  hand,  if  a  land  surface 
should  be  raised,  then  the  stream  and  its 
tributaries,  in  virtue  of  the  increased  gra- 
dient, would  begin  to  cut  actively  and  cor- 
rade  their  channels.  Features  of  former 
mature  topography  may  still  be  recog- 
nized, yet  now  through  the  uplift  we  find 
the  river  exhibiting  the  characters  of 
youth.  When  this  has  happened  a  river  is 

Fig.  53  -Drowned  and  dis-  gaid   to   be   revived,      Thus   the    streams    of 
membered rivers.  A,  estuary,          ,.  .__          _.      .        ..         .  .  • 

drowned  river;  B,  dismem-  southern  New  England,  which  is  an  up- 
bered  tributaries;  c,  remade  raised  former  peneplain,  have  again  been 
alluvial  land.  ge^  &^  WOrk  and  are  now  excavating  their 

valleys.    See  Figs.  54  and  55. 


Fig.  54.  —  A  revived  river.  Above,  the  gentle  slopes  of  the  former  matured  valley; 
below,  the  new  trench  of  the  revived  stream.  In  the  distance  the  accordant  heights 
of  the  gently  modulated  topography  show  the  level  of  a  former  peneplain. 

Terraces.  —  In  the  flood-plains  of  revived  rivers  may  sometimes  be  found 
terraces,  similar  to  those  described  on  page  57,  whose  mode  of  formation  is 
of  interest  and  importance  in  that  it  involves  both  depression  and  elevation 


RAIN    AND    RUNNING    WATER 


73 


of  land  and  may  be  described  as  follows,  with  reference  to  the  diagrams  in 
Fig.  56. 

We  first  imagine  a  river  r  to  cut  out  a  valley  in  the  underlying  rocks  E, 
as  shown  in  section  in  A,  the  land  standing  at  a  definite  level.  If  the  land 
is  now  depressed  so  that  the  sea  enters  the  valley,  it  will  be  drowned  and  the 
river  will  deposit  in  the  endeavor  to  raise  its  bed  to  baselevel.  When  this 
has  been  done  the  section  will  appear  as  in  B,  the  thickness  of  the  deposit 
G  indicating  the  amount  of  subsidence.  If  the  land  is  now  raised  to  a  new 


Fig.  55.  —  Valley  of  the  Deerfield  River,  Mass. 

svel,  as  in  C,  the  gradient  is  increased  and  the  revived  river  will  again  cut 
lown  to  a  new  baselevel  which  we  may  consider  to  be  at  r.  It  will  then 
riden  its  valley  by  meandering  and  lateral  planation,  as  previously  de- 
ribed,  and  while  this  is  going  on  the  remains  of  the  old  flood-plain  ti,  usually 
fronting  on  the  new  one  below  with  more  or  less  steep  bluffs,  will  form 
terraces.  The  height  of  the  bluffs  in  a  general  way  shows  the  amount  of 
upraise.  Although  terraces  may  be  formed  in  this  way,  they  are  more  gen- 
erally produced  by  other  causes,  the  most  important  of  which  has  already 
been  described.  They  are  a  noticeable  feature  in  the  valleys  of  many  New 
England  rivers. 


Fig.  56.  —  Illustrating  river-work  and  formation  of  terraces. 

horizontal. 


Vertical  scale  twice  the 


Incised  Meandering  Valleys.  —  If  we  imagine  a  land  reduced  to 
the  condition  of  a  peneplain,  or  nearly  so,  its  streams  would  wind 
sluggishly  on  their  way  to  the  sea,  meandering  over  well  graded 
plains  at  the  bottom  of  wide  shallow  valleys.  If  now  the  land 
should  be  raised,  the  streams  would  be  revived,  or,  as  sometimes 
said,  rejuvenated,  and  as  explained  in  the  previous  section,  set  at 
work  again  by  the  increased  gradient.  They  would  then  sink  their 


74  TEXT-BOOK    OF   GEOLOGY 

winding  courses  in  their  valley  floors  and  thus  form  new  incised 
meandering  valleys,  whose  depth  below  the  raised  upland  will  de- 
pend on  the  amount  of  the  elevation  and  the  rate  at  which  it  took 
place,  compared  with  the  speed  at  which  the  streams  have  been 
downcutting,  and  the  length  of  time  since  they  began  their  work. 
Such  sunken  meanders,  which  a  winding  stream  has  incised,  are 
spoken  of  as  being  entrenched.  A  view  of  an  incised  meandering 
valley  is  shown  in  Fig.  57. 


Fig.  57.  —  Incised  meandering  valley  of  the  San  Juan  River,  30  miles  below  Bluff, 
Utah;  canyon  1200  feet  deep.  H.  E.  Gregory,  Prof.  Paper  93,  U.  S.  Geol.  Surv. 
H.  H.  Vinson,  photo. 

The  rivers  of  northern  France  and  Belgium,  like  the  Meuse  and  the 
Moselle,  furnish  typical  examples  of  such  valleys.  The  head  waters  of  the 
Susquehanna  and  its  tributaries  in  Pennsylvania  also  present  illustrations  of 
them. 

Certain  features  which  these  valleys  may  exhibit  are  of  interest  in  con- 
nection with  river  work  and  they  may  be  understood  from  the  diagram  Fig. 
58,  and  from  what  follows.  A  river,  in  the  circumstances  mentioned,  is  not 
only  cutting  downward,  it  is  also  cutting  in  a  lateral  direction  against  the 
concave  sides  of  its  curves,  as  previously  explained  under  meanders,  which 
it  therefore  tends  to  enlarge.  Moreover,  this  lateral  cutting  may  be  ex- 
pected to  be  greatest  against  those  sides  of  the  curves  which  face  up  against 
the  general  course  of  the  river,  since  upon  them  the  weight  of  the  current 
must  come  with  attendant  corrasion.  These  sides  of  the  spurs  interlocking 
between  the  meanders  therefore  tend  to  be  "undercut/'  to  present  steep  and 
even  cliff-like  faces  to  the  course  of  the  river,  C,  C,  C,  Fig.  58.  The  other, 
down-valley  sides  of  the  spurs,  8,  S,  S,  on  the  contrary  are  apt  to  descend 
to  the  river  with  more  or  less  gentle  and  gradual  slopes,  often  covered  with 
sand  or  gravel  deposited  by  the  stream.  The  reason  for  the  difference  is, 
that  since  the  river  cuts  laterally  against  the  faces  C,  C,  and  also  vertically 
downward,  these  sides  of  the  spurs  are  being  eaten  into  and  consumed, 
whereas  it  tends  to  move  away  from  the  sides  S,  S,  and  with  slacker  current 
to  leave  deposits  on  them.  They  are  called  the  slip-off  slopes,  in  distinction 
from  the  undercut  ones,  C,  C,  since  the  stream  tries  to  slide  away  from  them 
without  eroding.  As  one  looks  down  the  course  of  such  a  valley  he  sees  only 
the  steep  and  wooded  or  cliff-like  faces  of  the  undercut  spurs,  which  give  it 


RAIN    AND    RUNNING    WATER 


75 


a  stern  and  lonely  aspect;  when  he  looks  up  the  valley  the  cultivated  and 
more  or  less  inhabited  fields  of  the  gentle  slip-off  slopes  may  confront  him. 

From  what  has  been  said  above  it  will  be  seen  that  an  entrenched  winding 
river  tends  to  become  more  circuitous  in  its  course,  and  that  the  whole 
system  of  meanders  moves  down  the  valley.  Further,  when  vertical  corrasion 
ceases  and  the  stream  becomes  graded,  it  will  begin  to  build  narrow  strips  of 
land  by  deposits  P,  P,  P,  Fig.  58,  along  the  slacker  current  sides  S,  S,  S,  known 
as  flood-plain  scrolls.  In  time,  as  the  valley  widens  by  lateral  planation 
(page  6  ),  these  scrolls  join,  and  a  complete  flood-plain  is  established. 

It  is  evident  that  as  the  meanders  enlarge  and  change  their  curves  they 
may  intersect,  forming  short  cuts,  as  shown  in  Fig.  48.  With  entrenched 


:  S 

res 


Fig.  58.  —  An  incised  meandering  valley;  arrows  point  downstream.  C,  C,  C,  "under- 
cut" slopes  of  valley;  S,  S,  S,  slip-off  slopes;  P,  P,  P,  flood-plain  scrolls.  After 
W.  M.  Davis. 

meanders  it  may  happen  that  the  neck  of  land  connecting  the  spur  end  is 
actually  undercut  at  the  narrowest  place,  leaving  the  former  spur  remnant 
as  an  island  joined  to  the  mainland  by  a  natural  bridge,  under  which  the 
stream  runs  in  the  short  cut  it  has  made.  Such  a  bridge  modified  into  an 
arch  by  later  erosion  may  be  seen  in  the  frontispiece. 

Consequent  and  Subsequent  Rivers.  —  If  one  imagines  for  any 
reason  a  newly  formed  land  surface,  as  for  instance  an  upraised  sea- 
bottom,  it  seems  clear  that  this  would  have  natural  bulges  and  hol- 
lows, for  a  considerable  area  could  not  be  elevated  with  a  perfectly 
plane,  smooth  surface:  the  courses  of  this  new  land  would  be  de- 
termined by  its  natural  slopes  and  topography.  Rivers  originating 
in  this  way  are  called  consequent  rivers  because  their  courses  are 
consequent  upon  such  original  features  of  the  relief  of  the  land. 

ey  may  persist  in  a  region  long  after  its  original  topography  has 
been  greatly  changed  by  erosive  processes  having  cut  away  the  less 
resistant  areas  of  rocks  more  rapidly  than  the  harder,  stronger  ones. 


76 


TEXT-BOOK    OF   GEOLOGY 


On  the  other  hand,  as  time  goes  on,  new  drainage  channels  may 
appear,  not  dependent  on  the  original  topography,  but  determined 
by  erosion  acting  differently  on  underlying  rock  areas  according  to 
their  resistance,  structure,  etc.,  as  illustrated  in  Figs.  59  and  60. 
Rivers  formed  in  this  way  are  called  subsequent.  A  consequent  river 
then  is  one  whose  course  has  been  directed  by  the  original  natural 
slopes  of  the  land  which  it  drains,  whereas  a  subsequent  stream  is 
one  occupying  a  valley  which  it  has  excavated  by  later  erosion  in 
harmony  with  underlying  rock  structures.  Such  subsequent  streams 
are  apt  to  be  tributaries  to  more  important  consequent  ones. 

In  the  Appalachian  region  some  of  the  large  rivers,  such  as  the  Delaware, 
Susquehanna  and  Potomac,  are  consequent  streams.  They  appear  to  have 
originated  on  an  old  peneplain  which  was  upraised,  and  although  subsequent 
erosion,  with  lowering  of  the  surface,  has  etched  out  a  whole  series  of  moun- 
tain ridges  athwart  their  paths  to  the  sea,  they  have  persisted  in  their  general 


Fig.  59.  —  Illustrates  consequent  rivers 
A  A  on  a  natural  slope,  newly  exposed; 
underground  structure  not  yet  re- 
vealed. 


Fig.  60.  —  With  lowering  of  surface  by 
erosion  in  Fig.  59,  a  hard  ridge  of  rock 
etches  out;  one  consequent  river,  A, 
persists  and  becomes  a  master  stream; 
the  other  drainage  is  diverted  and 
forms  a  subsequent  river  B. 


courses,  cutting  gaps  through  the  ridges,  and  only  here  and  there  being  some- 
what diverted  by  these  barriers.  They  have  thus  become  master  streams, 
and  receive  the  waters  of  a  great  number  of  subsequent  tributaries,  whose 
valleys  have  been  eroded  along  belts  of  soft  rocks  lying  between  the  harder 
ridges  crossed  by  the  master  streams.  See  Fig.  60. 

Antecedent  Rivers.  —  During  the  long  life  of  a  river  it  may 
happen  that  an  upwarping  of  the  land  may  take  place  athwart  its 
course.  If  the  river  has  a  gradient  which  will  give  it  the  requisite 
energy,  and  is  cutting  sufficiently  fast,  it  may  be  able  to  saw  down 
its  channel  through  the  upwarp  in  measure  as  it  rises,  and  thus  main- 
tain its  course.  A  stream,  whose  course  has  been  thus  determined  by 
a  previous  topography  and  does  not  now  conform  to  the  present  relief 
of  the  land,  is  called  an  antecedent  river.  Thus  the  Columbia  is 
held  to  have  cut  its  gorge  across  the  mountains  which  try  to  bar  its 
way,  and  the  Great  Kanawha  River  in  its  course  across  the  up- 
raised plateau  in  West  Virginia  has  preserved  its  original  winding 


RAIN    AND    RUNNING    WATER  77 

course.  Similar  instances  are  thought  to  occur  in  other  parts  of  the 
world,  as  with  some  of  the  rivers  of  the  Alps.  In  each  case  the  river 
is  thought  to  be  older  than  the  elevations;  otherwise  we  could 
scarcely  understand  how  such  drainage  ways  could  occur. 

Superimposed  Rivers.  —  It  sometimes  happens  that  consequent 
rivers  have  had  their  courses  determined  by  natural  features  of 
relief  on  a  new  land  surface  consisting  of  materials  of  a  certain  kind. 
The  courses  thus  determined  may  be  persisted  in  by  the  streams, 
although,  as  erosion  progresses  and  they  continue  to  sink  their 
channels,  they  may  be  compelled  to  do  so  in  rocks  and  rock  struc- 
tures of  a  very  different  nature  from  those  at  the  surface.  Finally, 
erosion  may  strip  off  the  overlying  material  entirely,  and,  with  the 
topography  etched  out  from  the  underlying  rocks,  the  old  stream 
channels  may  appear  as  quite  inharmonious.  This  is  illustrated  in 
.  61.  A  river,  whose  course  has  been  thus  predetermined  and 

rhich  is  not  now  in  adjustment  with  the  general  topography  and 

>ck  structures,  is  called  superimposed. 

In  this  case  its  smaller  tributaries  are  mostly  subsequent  streams.     Care 
lould  be  taken  not  to  confuse  antecedent  and  superimposed  rivers.     Both 


Fig.  61.  —  Illustrating  the  origin  of  a  superimposed  river.  A,  course  determined  by 
natural  slope  on  a  layer  of  sand  and  gravel;  B,  latter  removed  by  erosion  and 
river  pursuing  its  course  without  regard  to  underlying  rock  structure. 

may  cut  gorges  through  elevations  which  lie  as  barriers  athwart  their  courses, 
but  in  the  antecedent  the  elevation  has  risen  through  interior  forces  during 
the  life  of  the  river,  while  in  the  superimposed  it  has  been  produced  in  the 
general  lowering  of  the  surface  by  erosion,  through  some  rock  masses  being 
more  resistant  than  others,  that  is,  by  differential  erosion. 

The  smoother  the  underlying  rock  surface  is,  the  thinner  the  overlying 
mantle  of  material  may  be  to  produce  this  result.  If  an  irregular  surface 
of  rocks  of  varying  kinds  of  hardness  and  resistance  were  planed  down 
almost  to  baselevel  and  then  elevated,  the  drainage  would  be  rejuvenated  and 
consequent;  some  master  streams  would  persist  in  their  course,  and,  although 
their  tributaries  through  differential  erosion  would  be  subsequent  (page  76), 
they  would  have  the  characters  of  superimposed  streams,  even  if  a  mantle 
of  overlying  material  may  have  been  practically  absent  at  the  time  of  uplift. 

The  master  streams  of  the  Appalachian  region,  such  as  the  Delaware,  Sus- 
quehanna  and  Potomac,  have  the  characters  of  superimposed  rivers.  This  is 


78  TEXT-BOOK    OF    GEOLOGY 

well  illustrated  on  the  Delaware  in  the  fine  gorge  it  has  cut  through  the 
Kittatinny  or  Blue  Mountains,  a  barrier  which  erosion  has  etched  out  across 
its  way. 

Summary  of  River  Work.  —  From  what  has  been  stated  in  the 
preceding  pages  we  see  that  rivers  have  a  double  function ;  they  both 
erode  and  transport.  The  work  of  erosion  is  chiefly  done  in  the 
upper  steeper  parts  of  their  courses;  lower  down,  on  their  plains 
and  at  their  mouths,  the  work  is  largely  one  of  deposit  and  there- 
fore constructional.  Their  greatest  work  is  the  transporting  of  the 
material  furnished  them  by  general  erosion.  Finally,  we  have  seen 
that  they  have  a  life  history  which  passes  from  youth  into  maturity, 
and  on  into  old  age,  and  that  this  life  history  may  be  lengthened 
by  rejuvenation.  Some  features  of  stream  work,  such  as  the  nature 
of  their  deposits  and  other  ways  in  which  they  may  modify  the 
relief  features  of  the  land,  will  be  treated  later. 


w 

m< 

: 


CHAPTER  III 
LAKES  AND  INTERIOR  DRAINAGES 

Lakes  are  enclosed  bodies  of  water,  either  still,  or  with  but  a 
gentle  current;  they  are  usually  of  fresh  water,  but  many  lakes, 
and  some  very  large  ones,  are  saline.  Fresh-water  lakes  have  an 
outlet  and  in  some  ways  may  be  considered  as  expansions  of  streams. 

hile  lakes  are  found  in  all  parts  of  the  world  they  are  more  com- 
mon in  northern  and  in  mountainous  regions ;  the  reason  for  this  is 
explained  under  glaciers.  Lakes  are  caused  by  obstructions  to 

ainage,  and  such  obstructions,  or  lake  basins,  may  be  formed  in  a 
great  variety  of  ways,  some  of  the  more  important  of  which  are  as 
follows: 

Origin  of  Lake  Basins. —  (a)  Depressions  caused  either  by 
warping  of  the  earth's  crust,  or  by  its  breakage  and  displacement  in 
huge  blocks.  The  first  is  supposed  to  be  the  main  cause  of  Lake 
Superior,  while  to  the  second  is  due,  in  part  at  least,  the  basins  of  the 
large  lakes  in  central  Africa.  Some  smaller  lakes  in  the  western 
United  States,  such  as  Abert  Lake  in  Oregon,  have  been  formed  by 
such  displacements  of  the  earth's  crust.  The  warping  of  river  val- 
leys may  produce  lakes,  such  as  Lake  Temiskaming  in  Ontario. 
Some  of  the  largest  lakes  in  the  world  belong  in  this  general  class. 

(b)  Rock-basins  which  have  been  excavated  by  some  means.  The 
most  common  agency  for  these  is  the  action  of  glaciers,  as  will  be 
explained  later;  such  glacial  lakes  are  found  in  northern  regions, 
and  especially  in  high  mountains;  frequently  they  are  inconsider- 
able in  size.     Some  lakes  which  fill   crater-pits  due  to  volcanic 
action,  like  those  in  central  Italy,  also  belong  in  this  class. 

(c)  Lakes  due  to  natural  dams  which  have  been  formed  across  the 
drainage  channels  of  streams.    Such  dams  may  have  resulted  from 
various  agencies ;  they  may  be  made  by  streams  of  lava,  by  accumu- 
lations of  volcanic  ashes,  or  by  land-slides,  to  suggest  examples. 
Most  commonly,  however,  they  are  made  of  loose  material  such  as 
sand  and  clay  deposited  by  streams,  or  earth  and  stones  left  by  the 
moving  ice  of  glaciers.    Thus  a  swift  tributary  may  deposit  more, 
and  coarser,  material  in  a  slowly  moving  main  river  than  the  latter 

79 


gO  TEXT-BOOK    OF   GEOLOGY 

can  carry,  whereupon  a  dam  will  be  formed,  and  the  larger  stream 
expanded  into  a  lake  above  this  point,  as  the  Mississippi  is  changed 
into  Lake  Pepin  by  the  dam  made  by  the  Chippewa  River.  Where 
streams  enter  the  sea  through  estuaries,  the  ocean  waves  may  throw 
up  barrier  dams  across  their  mouths,  converting  them  into  fresh- 
water lakes.  Examples  of  this  are  numerous  along  the  Atlantic 
coast.  Perhaps  the  larger  number  of  lakes,  which  spangle  the 
surface  of  the  northern  part  of  the  country  and  lend  charm  to  the 
scenery  of  New  England,  the  Adirondacks,  Minnesota  and  Canada, 
are  due  to  dams  in  valleys  left  by  glaciers.  The  way  in  which  these 
dams  are  made  by  waves  and  by  glaciers  is  considered  in  detail  in 
other  places. 

Relic  Lakes.  —  Some  lakes  were  once  arms  of  the  sea  which  have  been  cut 
off  from  it  by  natural  dams  formed  by  an  upraise  of  the  land,  or  by  material 
deposited  by  some  agency,  such  as  the  delta  of  a  river.  Subsequently,  the 
rivers  running  into  them  have  rinsed  the  salt  out  and  they  have  become 
fresh-water  lakes.  Lake  Champlain  is  an  example  of  this.  Such  bodies  of 
water  have  been  recently  named  relic  lakes,  and  their  former  connection 
with  the  ocean  is  shown  by  the  marine  forms  of  life  still  living  in  them, 
often  greatly  modified  in  structure  and  habits  by  the  changed  conditions. 

Functions  of  Lakes.  —  Several  important  geological  functions 
are  performed  by  lakes:  they  regulate  the  flow  of  streams  with 
which  they  are  connected,  and,  by  acting  as  storage  reservoirs,  pre- 
vent disastrous  floods;  if  large,  they  tend  to  equalize  the  temper- 
ature of  the  country  surrounding  them,  cooling  the  air  in  summer 
and  warming  it  in  winter.  But  the  most  important  geological 
function  they  perform  is  in  acting  as  settling  basins  for  the  sedi- 
ment of  river  waters.  In  this  way  great  accumulations  of  trans- 
ported material  are  made  and  river  waters  are  clarified.  This  is 
strikingly  illustrated  in  Lake  Geneva  in  Switzerland,  into  the  upper 
end  of  which  the  Rhone  pours  as  a  thickly  turbid  stream,  while  from 
the  lower  end,  at  the  city  of  Geneva,  it  issues  as  a  river  of  beauti- 
fully clear  blue  water.  Its  deposited  sediments  have  made  a  delta 
six  to  seven  miles  long  at  the  upper  end  of  the  lake. 

In  lakes,  especially  the  larger  ones,  may  be  seen  the  geological 
work  of  waves  and  of  currents  along  shores.  These  are  best  studied, 
however,  in  connection  with  the  seacoast,  where  they  are  much 
more  strikingly  displayed. 

The  Duration  of  Lakes.  —  Considered  from  the  geological  stand- 
point lakes  are,  in  general,  only  temporary  affairs.  They  are  short- 
lived, since,  for  reasons  previously  mentioned,  the  rivers  which 
maintain  them  must  in  time  fill  them  up  with  sediment,  convert  them 


LAKES    AND    INTERIOR    DRAINAGES  81 

into  river  flats  or  alluvial  plains,  and  thus  obliterate  them.  And  also, 
since  the  river  gradient  has  a  sharper  slope  where  it  leaves  the  lake, 
the  channel  must  deepen  and  wear  backward  upstream  faster  at 
this  point,  and  this  may  cut  through  the  barrier  and  drain  the  lake 
before  it  is  filled.  Both  filling  and  draining  may  cooperate  to  de- 
stroy the  lake,  but  since,  in  general,  water  leaving  a  lake  is  clear  and 
has  little  power  to  erode,  filling  must  be  the  chief  factor.  In  humid 
regions,  however,  as  will  be  shown  later,  this  filling  may  be  done 
quite  as  much  by  deposits  of  organic  life  as  by  transported  sedi- 
ments, or  even  more. 

Yellowstone  Lake  was  once  of  greater  size,  and  drained  south  and  west  into 
Snake  River  and  the  Pacific.  The  present  Yellowstone  River,  then  a  much 
smaller  stream,  but  working  rapidly  backward  on  a  steep  gradient,  tapped  the 
lake,  partly  drained  it,  and  continues  to  divert  its  waters  into  the  Missouri 
and  so  into  the  Atlantic. 

Lakes,  therefore,  are  considered  to  be  indicative  of  topographic 
youth  and,  in  general,  this  is  true.  Thus  Florida,  which  is  a  sea- 
bottom  raised  in  a  recent  geological  period,  still  contains  many 
shallow  lakes  on  its  very  flat,  slightly  irregular  surface,  while  the 
other  Southern  States,  which  are  geologically  old  and  have  a  mature 
topography,  are  destitute  of  lakes,  save  those  made  by  the  Mis- 
sissippi and  its  tributaries  in  wandering  on  its  flood-plains.  The 
many  lakes  in  the  Northern  States  and  in  Canada  are  also  the 
result  of  a  new  surface  given  the  land  during  the  recent  ice  age. 

The  larger  lakes  in  Florida  are  probably  consequent,  due  to  the  flooding 
of  natural  shallow  basins  in  the  upraised  surface,  but  many,  possibly  most, 
of  the  very  great  number  of  small  lakes  and  ponds  appear  to  be  subsequent, 
in  that  they  occupy  hollows  leached  out  after  the  elevation  by  solution  of 
the  underlying  limestone. 

The  lakes  of  glacial  origin  in  the  north  may  fill  depressions,  either  ground 
out  in  the  country  rock  by  the  moving  ice,  or  left  in  the  irregular  surface 
of  the  sheet  of  debris  deposited  when  it  melted,  or  they  may  be  due  to  the 
deposits  forming  dams  in  valleys,  as  will  be  considered  later  under  glaciers. 

There  are  exceptions  to  this  general  rule  that  lakes  are  short- 
lived and  therefore  recent.  The  great  size  of  some  lakes,  such  as 
lose  forming  the  group  of  the  Great  Lakes,  often  combined  with 
special  geological  events,  as  in  the  case  of  Great  Salt  Lake,  may 
irve  to  prolong  the  lives  of  some  past  the  period  when  we  should 
normally  have  expected  them  to  disappear. 

But  although  lakes  must  eventually  be  obliterated  by  filling,  or 
draining,  or  both,  there  is  a  considerable  difference  in  their  final  his- 


82  TEXT-BOOK    OF   GEOLOGY 

tory,  and  also  in  their  nature,  dependent  on  the  climate  of  the  region 
in  which  they  are  situated,  and  this  demands  consideration. 

Lakes  in  Humid  Regions.  —  In  those  places  where  the  rainfall 
exceeds  the  amount  evaporated  from  the  surface  of  standing  water 
annually,  all  depressions  will  fill  up  and  become  lakes,  provided 
there  is  no  underground  drainage.  Such  lakes  must  have  an  outlet, 
and  will  overflow,  and  therefore  through  change  in  the  water  will 
remain  fresh.  And,  although  normally  they  must  disappear 
through  filling  with  sediments,  as  they  become  shallow,  or  if  they 
were  originally  small,  the  process  is  greatly  hastened  by  accumula- 
tion of  black  carbonaceous  matter  called  peat,  resulting  from  the 
decay  of  various  forms  of  plant  life,  and  through  this  peat  they 
become  converted  into  marshes,  bogs,  and  swamps  as  the  final  stage 
of  destruction.  This  process,  and  other  details  regarding  swamps, 
are  described  in  the  chapter  devoted  to  organic  agencies. 


Fig.  62.  —  A  temporary  lake,  in  a  semi-arid  region.     Wyoming.     G.  I.  Adams,  U.  S. 

Geol.  Surv. 

Lakes  in  Arid  Regions ;  Inland  Drainage.  —  The  amount  of 
rainfall  received  by  any  region  is  dependent  on  the  nature  of  the 
prevailing  winds  and  on  the  topography  of  the  country.  In  all  con- 
tinents there  are  areas  which  receive  so  little  moisture  that  they  are 
arid,  or  even  desert,  in  character.  Thus  in  North  America  the 
country  lying  between  the  Sierras  and  the  Wasatch  Range,  and 
mainly  in  Utah  and  Nevada,  which  is 'known  as  the  Great  Basin, 
has  a  very  light  rainfall  because  the  prevailing  winds  coming  from 
the  west  and  the  Pacific  have  most  of  their  moisture  discharged  by 
the  mountain  ranges  of  California  and  Oregon  before  reaching  it. 
In  such  regions  the  evaporation  may  greatly  exceed  the  rainfall; 
springs  are  rare,  streams  infrequent  and  of  scanty  volume  in  respect 


LAKES    AND    INTERIOR    DRAINAGES 


83 


to  the  size  of  the  valleys  which  they  drain;  many  depressions  in 
these  districts,  which  in  humid  climates  would  form  permanent 
lakes,  are  dry,  or  only  occasionally  filled  in  times  of  storms,  see 
Fig.  62. 

Surrounding  such  regions  are  stretches  of  country,  often  moun- 
tainous, which  receive  a  greater  rainfall  and  whose  streams  in  part 
are  directed  into  these  arid  tracts.  On  reaching  depressions  they 
fill  them  in  part  and  form  lakes  whose  size  is  dependent  on  a  nicely 
balanced  adjustment  between  the  amount  of  water  received  from 
the  river  and  that  lost  by  evaporation  from  the  surface  of  the  lake. 
Should  the  depression  be  small  the  river  may  fill  it  and  pass  on,  but, 


Fig.  63.  —  Alkaline  salt  lake  near  Parma,  Cc 


C.  E.  Siebenthal,  U.  S.  Geol.  Surv. 


as  is  very  frequently  the  case,  the  amount  evaporated  may  equal  the 
inflow  and  the  drainage  system  will  then  end  in  the  lake.  Such 
lakes  vary  in  size  during  different  parts  of  the  year,  or  from  one  pe- 
riod of  years  to  another,  in  response  to  fluctuations  in  the  rainfall 
and  in  the  water  discharged  into  them  by  the  incoming  streams.  It 
may  also  happen  that  a  river  running  into  such  a  region  may  dwindle 
so  much  from  evaporation,  before  reaching  a  depression  suitable  for 
the  formation  of  a  lake,  as  to  entirely  disappear.  River  systems 


84  TEXT-BOOK   OF   GEOLOGY 

like  these,  which  end  through  evaporation  in  interior  basins  without 
reaching  the  sea,  are  termed  interior  drainages. 

In  arid  regions  the  sudden  heavy  storms  which  sometimes  occur  give  rise 
to  floods  of  water,  which  rushes  down  through  channels  to  the  plains  below, 
where  it  may  spread  out  in  wide  thin  sheets.  Such  sheet  floods  make  tem- 
porary, extended,  shallow  lakes,  known  as  playa  lakes  for  the  reason  that 
the  deposits  spread  out  by  the  muddy  water  form  monotonously  level  plains 
called  play  as. 

Salt  Lakes.  —  It  has  been  already  explained  in  the  description 
of  the  river's  burden  that  a  part  of  it  consists  of  various  salts  in 
solution,  and  that  such  salts  are  carried  by  all  streams,  even  if,  in  a 
given  volume,  the  water  appears  so  fresh  that  they  can  only  be  de- 
tected by  chemical  means.  In  ordinary  rivers  these  salts  are  dis- 
charged into  the  sea,  but  in  interior  drainages,  since  they  cannot  be 
dissipated  by  evaporation  like  the  water,  they  must  constantly 
accumulate  at  the  point  where  the  drainage  ends. 


I ... 

-.**- **-, 


i 


Fig.  64.  —  Alkali  deposit  on  the  shore  of  Soda  Lake,  Parma,  Colo.     C.  E.  Siebenthal, 

U.  S.  Geol.  Surv. 

If  the  river  ends  by  dwindling,  its  lower  part  finishes  in  a  stretch 
covered  with  salt  deposits,  sometimes  in  wet  seasons  converted  into 
a  salt  marsh  or  shallow  lake,  and  known  as  a  salina.  Examples 
of  these  are  found  in  the  Tarim  River  which  ends  in  the  Desert  of 
Gobi  in  central  Asia,  in  the  Desaguedero  River  which  carries  the 
drainage  from  Lake  Titicaca  in  Bolivia,  and  in  many  other  places. 
But  if  the  end  of  the  drainage  system  is  a  lake  the  latter  is  bound  in 
time  to  become  salt  through  the  concentration  of  these  substances, 
and  such  salt  lakes  are  features  of  arid  or  desert  regions  in  all  the 


LAKES    AND    INTERIOR    DRAINAGES 


85 


continents.  Examples  of  them  are  the  Dead  Sea  in  Palestine,  Lake 
Van  in  Armenia  and  the  Aral  Sea  in  Siberia,  Lakes  Shirwa  and 
Rudolph  in  Africa,  Lakes  Eyre  and  Torrens  in  Australia,  and  Lake 
Chiquita  in  South  America.  In  North  America  the  best  ones  are 
found  in  the  Great  Salt  Lake  in  Utah,  Pyramid  Lake  and  others  in 
Nevada,  and  Mono  Lake  in  California. 

It  is  only  the  final  lake  in  an  interior  drainage  system  which  becomes  salt; 
an  intermediate  lake  remains  fresh  because  its  waters  are  changed.  Thus 
Utah  Lake  which  flows  into  Great  Salt  Lake,  Lake  Tahoe  which  runs  into 
Pyramid  Lake,  and  the  Sea  of  Galilee  which  discharges  into  the  Dead  Sea 
are  all  fresh,  whereas  the  terminal  in  each  case  is  salt. 

Great  Salt  Lake  has  an  area  of  about  2000  square  miles ;  its  average  depth  is 
about  20  feet.  The  water  is  a  strong  brine,  being  five  or  six  times  as  salt  as 
that  of  the  ocean,  which  in  composition  it  closely  resembles;  its  salinity  is 
about  18  per  cent,  and  the  buoyancy,  from  the  increased  specific  gravity,  is  so 
much  higher  than  ordinary  fresh  water  that  one  floats  upon  it  almost  like  a 
cork.  The  chief  salts  are  common  salt  (sodium  chloride)  and  sodium  sulphate; 


Fig.  65.  —  Islands  of  calcareous  tufa  in  Pyramid  Lake,  Nevada.     I.  C.  Russell,  U.  S. 

Geol.  Surv. 


of  the  former  Gilbert  estimated  the  lake  to  contain  400,000,000  tons,  of  the 
latter  30,000,000  tons.    Calcium  carbonate,  which  is  brought  in  by  the  inflow- 
ing waters,  is  deposited  as  a  granular  sand  on  the  bottom  and  shores. 
The  waters  of  Great  Salt  Lake  have  receded  in  recent  times  owing  to  the 


86 


TEXT-BOOK  OF  GEOLOGY 


diversion  of  the  Jordan  and  Bear  rivers,  which  maintain  it,  for  purposes  of 
irrigation,  and  the  consequent  evaporation  of  a  part  of  the  supply  on  land. 
In  the  last  few  years,  however,  it  appears  to  be  again  expanding,  owing, 
possibly,  to  an  increase  in  rainfall  and  diminished  evaporation. 

Salt  and  Alkaline  Lakes.  —  In  Great  Salt  Lake,  like  many  others  of  its 
type,  chlorides  and  sulphates  are  the  chief  salts,  but  in  some  lakes,  as  in 
Pyramid  Lake  in  Nevada  and  its  neighbors,  there  are  in  addition  notable 
quantities  of  the  carbonates  of  soda  and  lime  and  their  waters  give  an 
alkaline  reaction.  We  may  therefore  distinguish  between  these  cases  and 
speak  of  salt  and  alkaline  lakes,  although  in  western  America  all  natural  salts, 
either  as  deposits  on  the  land,  or  in  water,  are  commonly  and  incorrectly 
spoken  of  as  "alkali,"  see  Fig.  64.  The  reason  for  this  difference  appears  to  be 
that  the  water  of  Great  Salt  Lake  drains  from  an  area  chiefly  occupied  by 
rocks  that  were  once  laid  down  as  sediments  on  the  sea  floor;  on  being  raised 
to  form  land  they  brought  up  in  their  pores  the  sea  salts  which  are  now 

being  leached  out,  while  the 
Nevada  basin  is  largely  cov- 
ered by  igneous  rocks,  lavas, 
etc.,  destitute  of  sea  salts 
and  largely  composed  of 
feldspar,  whose  decay  yields 
carbonates  as  explained  un- 
der the  formation  of  soil, 
page  27.  In  the  alkaline 
lakes  the  carbonates,  espe- 
cially lime  carbonate,  are 
deposited  as  calcareous  tufa 
(see  page  167)  in  many 
striking  and  curious  forms 
encrusting  the  enclosing 
rocks  of  the  basin,  in  some 
places  in  huge  masses.  See 
Fig.  65. 


Detached  Salt  Lakes. 
—  In  some  cases  salt 
lakes  are  known  to 
have  been  formed  by 
arms  of  the  sea  having 
been  detached  from  the 
main  ocean  by  the  rais- 
ing of  some  intervening 
barrier,  such  as  sand 
ridges  thrown  up  by  the 

waves   and   winds   pro- 
Fig.  66.-Map  of  the  former  Lake  Bonneville;        ducing  them  Qn  &  gmall 
lined  areas  show  present  water-bodies.  , 

scale,  or  an  upraise  of 


LAKES    AND    INTERIOR    DRAINAGES 


87 


the  earth's  crust  on  a  large  one.  Or  they  may  have  been  made  by 
dams  formed  by  rivers,  glaciers  or  other  agencies.  In  a  humid  cli- 
mate these  would  become  rinsed  out  and  fresh,  and  thus  turned  into 
relic  lakes,  page  80,  but  in  an  arid  region  they  may  either  dry  up  and 
disappear,  or  become  the  final  evaporating  terminus  of  an  inland 
drainage  and  thus  persist. 

The  Caspian  Sea  is  one  of  the  best  known  examples  of  this.  It  receives  the 
waters  of  the  Volga  and  other  rivers  which  are  building  deltas  and  slowly  filling 
it.  Its  water  has  a  composition  similar  to  that  of  the  sea,  but  is  somewhat 


Fig.  67.  —  Former  shore  lines  and  wave-cut  terraces  of  the  ancient  Lake  Bonneville. 

fresher,  because  a  large  gulf  on  its  eastern  side  with  narrow  inlet  is  acting  as  the 
final  evaporating  pan,  and  in  it  the  salts  are  being  concentrated  and  de- 
posited. It  is  therefore  being  slowly  freshened,  and  is  turning  into  a  relic 
lake.  Its  former  connection  with  the  sea  is  shown  by  the  chemical  similarity 
of  the  salts  in  its  waters  and  by  the  nature  of  the  animal  life  it  contains, 
seals,  for  example,  being  found  in  it.  It  is  believed  to  be  the  remnant  of  a 
great  arm  of  the  ocean  which  once  stretched  northward,  over  what  are  now 
the  steppes  of  Russia,  to  the  Arctic  Ocean. 

History  of  Salt  Lakes.  —  The  study  of  salt  lakes  and  their  sur- 
roundings reveals  the  fact  that  in  many  cases  they  are  merely  the 
shrunken  remnants  of  much  greater  bodies  of  water  that  once 
occupied  their  basins.  Thus  Great  Salt  Lake  is  the  remnant  of  an 
inland  sea  as  large  as  Lake  Michigan  f&bout  20,000  square  miles) 
and  nearly  twice  as  deep,  to  which  the  name  of  Lake  Bonneville  has 
been  given,  see  Fig.  66,  while  Pyramid  Lake  and  its  neighbors  in 
Nevada  are  the  pools  remaining  from  a  great  lake  nearly  as  large  as 


88 


TEXT-BOOK   OF   GEOLOGY 


Lake  Erie,  which  has  been  called  Lake  Lahontan.  The  evidence  for 
this  is  found  high  up  on  the  slopes  of  the  basins  where  the  edge  of 
the  old  water  surface  is  shown  by  the  line  of  wave-cut  terraces, 
bars,  and  beaches,  see  Fig.  67,  such  as  are  described  later  under  the 
work  of  the  sea,  and  in  the  deltas  and  bars  made  by  the  incoming 
rivers,  and  now  seen  lying  on  the  desert  floors  surrounding  the  pres- 
ent water  bodies,  which  were  the  former  lake  bottoms.  The  evidence 
further  shows  that  these  lakes  were  once  rilled  up,  and  then  entirely 
dried  away ;  were  filled  a  second  time  and  then  dried  down  to  their 
present  condition,  these  variations  having  been  dependent  on  great 
climatic  changes  in  these  regions.  This  is  also  shown  by  the  deposits 
on  their  floors ;  first,  salt  when  the  lakes  dried  up ;  then  clays  washed 
in  when  they  refilled,  and  which  protected  the  salt  from  dissolving; 
and  then  another  layer  of  salt  when, they  again  dried  up.  This  dry- 
ing of  lakes  and  deposit 
of  salts  is  of  great  interest 
for  it  enables  us  to  under- 
stand the  presence  of  salt 
beds  in  the  rocks  and  con- 
nect them  with  dry  climates 
and  desert  conditions  in  the 
past  in  regions  which  now 
have  humid  climates  and  a 
very  different  character. 

In  this  connection  the  history 
of  the  Salton  Sea  is  instructive. 
The  Gulf  of  California  once  ex- 
tended far  beyond  its  present 
limits  into  California.  The  Colo- 
rado River,  discharging  into  this, 
built  its  delta  as  a  great  dam 
across  the  gulf,  shutting  off  its 
upper  portion  from  the  ocean 
and  converting  it  into  a  de- 
tached salt  lake.  The  river 
flowed  down  the  south  slope  of 
its  delta  into  the  gulf;  and  the 
lake,  thus  left  without  inflow  in 
an  arid  climate,  gradually  dried 
away,  leaving  a  vast  desolate 
salt-encrusted  basin,  known  as 

Imperial  Valley.  Recently,  in  the  attempt  to  divert  a  portion  of  the  Colorado 
out  on  the  northward  slope  of  the  delta  for  purposes  of  irrigation,  the  river 
in  a  period  of  flood  got  beyond  control  and  flowed  again  into  the  basin, 


M  E  X 
(LOWER 
CALIFOENI 


'ulfof 
California 


Fig.  68. —  Delta  of  the  Colorado  River, 
showing  distributary  channels,  canals 
and  Salton  Sea. 


LAKES    AND    INTERIOR    DRAINAGES  89 

partly  filling  it  up  and  forming  a  lake  450  square  miles  in  area  and  80  feet 
deep,  known  as  the  Salton  Sea,  see  Fig.  68.  After  great  expense  and  labor  the 
river  has  again  been  forced  back  into  its  seaward  channel,  and  the  lake,  thus 
accidentally  rejuvenated,  will  again  in  time  dry  up  and  disappear. 


CHAPTER  IV 
THE  OCEAN  AND  ITS  WORK 

General  Characters. —  The  ocean  covers  nearly  three  quarters 
of  the  globe.  We  are  apt  to  consider  its  surface  as  that  of  a  true 
sphere,  everywhere  the  same  distance  from  the  center,  and  to  use 
this  as  a  datum  plane  "sea-level,"  but  this  is  far  from  being  correct. 
Aside  from  the  fact  that  the  earth  is  not  a  true  sphere,  but  a 
spheroid  so  flattened  at  the  poles  that  the  polar  diameter,  on  which 
the  earth  revolves,  is  27  miles  less  than  one  in  the  plane  of  the  equa- 
tor, the  surface  is  also  distorted  by  the  waters  being  drawn  against 
the  continental  masses  by  their  gravitational  attraction.  Thus  the 
sea-level  is  higher  on  the  coasts  than  far  out  at  sea,  and  higher  on 
some  coasts  than  on  others  where  high  land-masses,  like  the  Andes, 
are  close  to  the  shore.  These  vertical  differences  of  level  are  so  small 
compared  with  the  vast  horizontal  scale  that  we  cannot  detect  them 
by  ordinary  observation. 

The  average  depth  of  the  sea  is  about  2J  miles  (13,000  feet), 
varying  somewhat  in  the  different  oceans.  The  relief  features  of 
the  globe  naturally  divide  into  two  great  classes,  continental  areas 


Land         Shore 


Epicontinental    Sea 


Sea  level 


Fig.  69.  —  Section  through  edge  of  continent  into  ocean  basin. 

and  ocean  basins;  the  amount  of  ocean  water  is  so  great,  that  not 
only  are  the  deep  basins  filled,  but  also  somewhat  overflowed,  so 
that  a  border  zone,  around  most  of  the  coasts  of  the  land  areas,  is 
covered  out  to  the  depth  of  about  600  feet.  These  slightly  sub- 
merged portions  of  the  continents  are  known  as  the  continental 
shelves  or  platforms,  and  it  has  been  estimated  that  over  10,000,000 
square  miles,  or  7  per  cent  of  the  ocean's  bottom,  forms  their  total 
area.  Those  parts  of  the  ocean  which  lie  upon  them  are  called 
epicontinental  seas  (epi,  upon,  or  above).  See  Fig.  69.  These  re- 
lations are  of  great  importance,  for,- as  will  appear  later,  the  conti- 
nental shelves  and  epicontinental  seas  have  been  in  the  past,  as 

90 


THE  OCEAN  AND  ITS  WORK  91 

they  are  at  present,  places  where  processes  and  results  of  profound 
geologic  significance  occur. 

Off  the  Atlantic  coast  of  North  America  the  continental  shelf  is  broad, 
about  100  miles,  or  so,  whereas  on  the  Pacific  side  it  is  narrow,  about  10  miles 
wide,  from  Mexico  northward  to  British  Columbia,  where  it  begins  to 
broaden.  California  and  Oregon,  therefore,  slope  quite  sharply  down  into 
the  Pacific  basin. 

The  ocean  bottom,  in  general,  is  monotonously  level,  and  without  the 
smaller  relief  features,  the  hills  and  valleys,  of  the  land.  Nevertheless,  on  a 
large  scale  there  are  swells  and  depressions  rising  above,  and  sinking  below, 
the  general  floors  of  the  oceans.  The  depressed  areas,  when  more  than  18,000 
feet,  are  known  as  deeps,  and  there  are  nearly  60  of  them.  Some,  narrow 
and  trough-like  in  form,  are  situated  near  the  margins  of  the  continental 
platforms;  others,  irregular  or  more  basin-like  in  nature,  are  in  the  central 
portions  of  the  ocean.  The  greatest  depth  in  the  Pacific,  40  miles  north  of 
Mindanao,  Philippine  Islands,  is  32,088  feet.  Near  the  coast  of  Japan  is  another 
deep  of  28,000  feet.  In  the  Atlantic  the  greatest  deep,  of  27,000  feet,  is  off  Porto 
Rico.  These  deeps  correspond  in  character  with  the  highest  elevations  of  the  land, 
up  to  nearly  30,000  feet  in  the  Himalaya  Mountains.  The  earth  and  the  ocean  are 
on  such  a  vast  scale,  as  compared  with  man,  that  it  is  difficult  to  realize  what  a  mere 
film,  relatively,  the  sea  is  upon  the  surface  of  the  globe.  If  a  ball  three  feet 
in  diameter  were  dipped  into  water,  and  withdrawn,  the  film  of  wetness  ad- 
hering to  it  like  a  skin  of  varnish  would  represent  the  ocean. 

It  is  quite  certain  that  sea-level  has  changed  very  often  during  past  ages, 
not  only  locally  by  the  relative  rise  and  fall  of  land  areas,  but  absolutely  by 
increase  in  the  actual  volume  of  water.  This  point  will  be  considered  further 
in  several  places,  where  it  is  a  matter  of  importance. 

Chemical  Composition.  —  The  chemical  investigations  which 
have  been  made  of  sea-water,  from  various  parts  of  the  world  and  at 
different  depths,  show  that  the  composition  of  the  ocean  is  remark- 
ably uniform.  A  large  part  of  the  known  chemical  elements  have 
been  detected  in  sea-water,  including  gold,  silver,  copper,  barium, 
strontium,  rubidium,  boron,  fluorine,  and  others,  but  most  of  these 
substances  are  present  in  such  minute  amounts  that  they  have  no 
practical  interest  or  geologic  importance.  The  percentage  of  salts  in 
sea-water  is  about  3J,  distributed  as  follows: 

100  Ibs.  of  sea-water  contain  3.5  Ibs.  of  salts,  and  100  Ibs.  of  these  salts  con- 
tain approximately, 

Lbs. 

Sodium  chloride,  NaCl 77.8 

Magnesium  chloride,  MgCl2 10.9 

Magnesium  sulphate,   MgSO4 4.7 

Calcium  sulphate,  CaSO4 3.6 

Potassium  sulphate,  K9SO4 2.5 

Calcium  carbonate,   CaCO3 0.3 

Minor    constituents 0.2 

Total..  ..1000 


92  TEXT-BOOK   OF   GEOLOGY 

From  this  it  is  seen  that  the  chlorides  and  sulphates  of  sodium, 
potassium,  calcium,  and  magnesium  are  the  main  substances,  with 
common  salt,  NaCl,  greatly  predominating.  In  addition  to  these 
salts,  sea-water  contains  dissolved  gases,  chiefly  air  and  carbonic 
acid  gas,  C02,  whose  amounts  vary  according  to  temperature  and 
depth,  and  accordingly  in  the  past  as  the  ocean  has  been  under 
glacial  or  warm  climates.  Roughly,  as  an  average,  we  may  say  that 
each  liter  of  sea-water  contains  about  20  cubic  centimeters  of  air, 
which  is  much  richer  in  oxygen  than  the  atmosphere,  and  about 
4^  hundredths  of  a  gram  of  C02.  The  importance  of  these  gases  is 
very  great,  for  upon  the  supply  of  oxygen  depends  the  life  of  the 
organisms  in  the  sea,  while  the  carbon  dioxide,  C02,  whose  total 
quantity  is  at  present  about  20  times  that  in  the  atmosphere,  acts 
as  a  regulator  of  the  amount  in  the  air  and,  since  the  average 
temperature  over  the  world  depends  in  part  on  the  carbon  dioxide 
contained  in  the  atmosphere,  variations  in  the  quantity  in  the  sea, 
and  therefore  in  the  air,  in  past  times  have  been,  as  we  shall  see 
later,  one  factor,  in  addition  to  others,  productive  of  great  changes 
in  climate  and  in  geological  processes. 

Functions  of  the  Ocean.  —  These  are  somewhat  similar  to  those 
which  have  been  mentioned  as  being  performed  by  lakes,  but  on  a 
vastly  greater  scale.  The  ocean  acts  as  a  regulator  of  climate  over 
the  world  through  the  great  currents  moving  in  it,  and  especially 
aids  in  equalizing  the  temperature  of  adjacent  land  areas;  through 
its  waves  and  tidal  currents  it  is  an  energetic  agent  of  erosion  which 
destroys  the  land;  it  is  the  final  settling  reservoir  in  which  are  de- 
posited the  sediments  brought  down  by  the  rivers,  as  well  as  those 
produced  by  its  own  erosion  of  the  coasts,  and  lastly  it  uses  this 
material  in  a  constructive  manner  in  the  production  of  islands  and 
other  features  peculiar  to  coast  lines.  All  of  these  functions  are 
worthy  of  attention,  but  since,  as  will  be  noted  from  what  is 
stated  above,  they  so  largely  depend  on  the  various  movements  of 
the  waters  of  the  ocean,  it  is  well  to  consider  the  latter  first.  For  if 
the  ocean  were  still,  inert,  it  would  have  little  effect  as  a  factor  in 
geological  work,  compared  with  what  it  now  performs.  Its  move- 
ments may  be  divided  into  three  great  classes :  ocean  currents,  tides 
and  tidal  currents,  and  waves. 

Ocean  Currents.  —  The  unequal  heating  of  the  ocean  by  the  sun 
in  tropical  and  polar  regions  would  establish  a  slow  general  circu- 
lation of  its  waters  through  convective  movements.  This  action, 
however,  is  controlled,  hastened  and  magnified  by  the  wind  belts  of 
the  earth  described  on  page  11,  and  by  the  disposition  of  land  and 


THE  OCEAN  AND  ITS  WORK 


93 


sea.  Driven  by  the  trade  winds,  there  is,  in  either  great  ocean,  in 
equatorial  regions,  a  broad  current  moving  westward,  along  the 
surface.  When  this  strikes  the  continental  coasts  it  divides,  one 
part  turning  northward,  the  other  southward,  and  each  circling 
returns  to  the  equatorial  belt,  thus  making  in  each  ocean  a  vast 
eddy,  one  north,  the  other  south,  of  the  equator.  In  the  same  man- 
ner there  is  a  circling  movement  in  the  Indian  Ocean.  These  main 
currents  are  shown  in  Fig.  70.  In  the  center  is  a  more  quiet  area, 
known  in  the  North  Atlantic  as  the  Sargasso  Sea.  When  these  broad 
slow  movements,  which  are  known  as  drifts,  approach  the  coast 
lines,  the  water  by  its  accumulation  and  the  configuration  of  the 
land  may  become  confined  and  hastened  in  its  motion,  giving  rise 
to  streams.  Thus  in  the  North  Atlantic  the  equatorial  current 
riking  the  north  coast  of  South  America  is  deflected  northward. 


^a\,^/«  i 


ve: 

: 

CO] 


Fig.  70.  —  Map  showing  main  ocean  currents  and  drifts. 

A  part  enters  the  Caribbean  Sea  and  the  Gulf  of  Mexico,  whence  it 
issues  through  the  greatly  confined  straits  of  Florida  and  passes 
northeast  into  the  Atlantic  as  the  well-known  Gulf  Stream.     Its 
velocity  as  it  comes  from  the  straits  is  nearly  100  miles  a  day,  but 
is  diminishes  as  it  approaches  mid-ocean,  and  as  it  grows  larger 
d  broader.    Finally  its  motion  sinks  to  10  miles  a  day  and  it  be- 
comes a  general  drift  of  the  ocean  waters  as  it  approaches  the  shores 
f  Europe.    Here  it  divides  and  one  part  turns  southward  to  pass 


94  TEXT-BOOK    OF   GEOLOGY 

along  the  coast  of  Africa,  and  so  to  join  the  westward  equatorial 
drift  again.  Another  portion  passes  northward  into  the  Arctic 
sea,  and  to  balance  this  a  cold  current  comes  down  from  the  coasts 
of  Greenland  around  Newfoundland  and  through  the  Straits  of 
Belle  Isle,  past  Nova  Scotia  and  New  England,  and  gradually  passes 
under  the  warm  surface  current  of  the  Gulf  Stream. 


In  a  similar  way  in  the  North  Pacific  a  current  turns  northward  and  east- 
ward and  then  turns  southward  along  the  western  coast  of  North  America.  It 
is  known  as  the  Japanese  current.  These  warm  currents  moving  into  northern 
latitudes  have  a  great  effect  upon  climatic  conditions  in  the  lands  whose 
shores  they  strike.  The  air  in  the  belt  of  westerlies,  page  11,  warmed  by  the 
water,  moves,  in  its  eastward  course,  inland  on  the  western  coasts  of  Europe 
and  North  America,  and  these  coastal  regions  therefore  enjoy  a  mild  and 
equable  climate.  But  in  the  same  latitude,  on  the  western  side  'of  the  At- 
lantic, Labrador  and  Newfoundland,  subjected  to  air  currents  which  have 
moved  eastward  across  the  continent,  have  a  harsh  continental  climate,  which 
the  cold  waters  of  the  returning  Polar  current  help  to  render  almost  sub- 
Arctic.  Thus  the  ocean  currents,  like  the  atmosphere,  in  taking  part  in  the 
general  circulation  on  the  surface  of  the  globe,  are  great  distributors  of  heat. 
They  carry  warmth  into  the  Arctic  seas  and,  returning  as  cold  currents,  they 
bring  with  them  its  ice  masses  to  be  melted  in  warmer  regions.  Were  it  not 
for  their  agency  ice  would  continually  accumulate  in  the  Polar  regions  and, 
while  in  general  they  perform  no  direct  geological  work,  indirectly,  in  what 
they  accomplish,  they  affect  geological  processes  and  are,  as  we  shall  see 
later,  of  great  importance.  In  some  places,  as  in  the  straits  of  Florida,  they 
may  scour  the  bottom,  but  such  action  is  infrequent  and  of  small  account 
compared  with  the  work  of  the  currents  next  to  be  considered. 

Tides  and  Tidal  Currents.  —  Without  going  into  elaborate  ex- 
planation it  may  be  briefly  stated  that  the  tide  is  an  uplift,  or  huge 
wave,  of  the  ocean  caused  by  the  attraction  of  the  moon  and,  to  a 
lesser  degree,  of  the  sun.  Were  there  no  continents  it  would  pass 
around  the  world  in  nearly  24  hours,  and  as  there  are  two  such  up- 
raises, one  on  the  side  of  the  earth  next  to  the  moon,  the  other  on 
the  side  opposite,  there  are  two  such  waves,  or  tides,  each  day.  In 
the  open  ocean,  its  height  is  so  low  and  its  base  so  vast,  that  it  is 
not  detectible  as  a  wave  in  passing  under  a  vessel ;  but  on  striking 
the  coasts,  as  it  does  every  12  hours,  it  first  piles  up  upon  the  shore 
and  then  recedes,  producing  the  familiar  phenomenon  known  as  the 
tides.  The  time  interval  between  tides  is  12  hours  and  26  minutes, 
and  this  interval  of  26  minutes  explains  why  the  time  of  high  and 
low  water  progressively  changes  each  day.  The  effect  of  the  tide 
on  the  coast  depends  greatly  on  the  configuration  of  the  latter;  on 
projecting  headlands  its  height  may  be  only  a  few  feet,  while  in 


THE   OCEAN   AND   ITS   WORK 


95 


Fig.  71.  —  Low  tide,  Port  Williams,  Nova  Scotia,  on  the  Bay  of  Fundy. 

narrow  bays  and  estuaries  it  may  pile  the  water  up  to  many 
times  this. 

The  J$&y  of  Fundy  on  the  Atlantic  coast  affords  one  of  the  best  examples 
known  of  the  cumulative  effect  of  the  tide  in  a  funnel-shaped  estuary.  At  its 
head  in  the  Basin  of  Minas  tides  of  30  to  40  feet  are  common,  while  heights 
of  50  feet  are  sometimes  reached.  The  difference  between  high  and  low  tide 
at  the  same  place  in  this  basin  is  strikingly  shown  in  Figs.  71  and  72. 


72.  —  High  tide,  Port  Williams  on  the  Bay  of  Fundy. 
variation  of  40  feet. 


Same  as  Fig.  71.     A 


96 


TEXT-BOOK    OF   GEOLOGY 


The  immense  bodies  of  water  moving  in  and  out  of  bays  and  estuaries,  and 
along  the  coast  every  six  hours,  produce  strong  tidal  currents,  which,  like 
rivers,  have  a  twofold  geologic  function  in  that  they  both  erode  and  trans- 
port. The  work  done  by  tides  in  scour  of  the  bottom  and  transportation 
of  material  along  some  coasts  is  very  great.  Often  in  entering  a  long  nar- 
row estuary,  at  the  turn  from  low  water,  the  incoming  tide  rushes  rapidly 
forward  in  an  immense  wave  or  series  of  waves  10  to  20  feet  in  height 
called  a  bore,  or  eagre.  Examples  are  seen  in  the  rivers  entering  the  head 
of  the  Bay  of  Fundy,  the  estuary  of  the  Severn  in  England,  the  Seine  in 
France,  the  Hoogly  in  India,  and  the  Tsientang  in  China.  See  Fig.  73. 
The  erosive  power  of  such  currents  is  very  great,  while  the  heavily  turbid 
condition  of  the  water  testifies  to  the  amount  of  sediment  transported. 
Although  minute  tides  may  be  detected  in  enclosed  seas  and  large  lakes, 
such  as  the  Black  Sea  and  Lake  Michigan,  they  are  too  feeble  to  be  of  im- 
portance, or  to  perform  geological  work. 

Waves.  —  These  are  due  to  the  impulse  of  the  wind.  The  water 
particles  may  be  considered  as  moving  in  circles  which  generate  the 


Fig.  73.  —  Bore  of  the  Seine  advancing  upstream. 

wave-form;  while  the  form  advances  the  water  does  not.  When 
waves  move  into  shallowing  water,  with  enlarging  circular  orbits  and 
decreasing  depth,  the  time  arrives  when  there  is  not  sufficient  water 
to  complete  the  wave-form  and,  as  a  result,  the  top  of  the  wave 
arching  over  is  unsupported  and  collapses,  the  water  being  given  a 
strong  forward  motion.  This  produces  the  breakers,  or  surf,  so 
common  a  feature  along  seacoasts.  The  distance  from  crest  to 
crest  is  the  length  of  the  wave,  and  that  vertically  from  trough  to 
crest  is  its  height.  The  length  of  average  storm  waves  in  the  North 
Atlantic  is  400  feet,  the  height  20  feet,  but  in  times  of  great  storms 
the  length  may  be  increased  to  1000  feet,  or  more,  and  the  height 
to  over  40  feet.  Storm  waves  with  breaking  crests  are  known  as 


(n 



THE  OCEAN  AND  ITS  WORK  97 

seas,  but  since  waves,  which  may  have  a  velocity  of  from  20  to  60 
miles  an  hour,  may  extend  far  beyond  the  storm  tract  which  gen- 
erated them,  they  may  lose  their  crests,  and  in  great  part  their 
height,  and  appear  as  long  heavy  undulations  of  the  surface  known 
as  ground-swells.  Both  ground-swells  and  seas  give  rise  to  breakers 
or  surf,  on  approaching  the  coast  line. 

There  appears  to  be  some  uncertainty  as  to  how  deep  the  in- 
fluence of  waves  on  material  lying  at  the  bottom  extends.  It  de- 
pends on  the  size  of  the  waves,  and  is  therefore  less  in  lakes  and 
enclosed  seas,  like  the  Mediterranean,  than  in  the  open  ocean. 
Probably  600  feet  represents  the  limit  at  which  fine  sand  is  dis- 
turbed off  the  Atlantic  coast,  but  at  from  60  to  100  feet  sand,  gravel 
and  even  pebbles  are  moved,  a  fact  of  importance  in  considering 

e  geological  work  done  by  the  waves. 


• 


The  water  thrown  on  the  shore  by  the  surf  returns  seaward  in  a  bottom 
rrent  called  the  undertow.  Where  waves  strike  a  shore  obliquely  the  run 
of  the  water,  due  to  successive  impulses,  generates  a  current  along  shore. 
While  this  may  be  too  feeble  in  itself  to  transport  material,  when  the  latter 
stirred  up  by  the  waves  is  held  in  suspension  and  carried  out  by  the  under- 
tow, the  littoral,  or  shore  current,  may  move  it  along.  It  is  by  this  com- 
bined action  of  waves  and  currents  made  by  waves  and  winds  that  material 
is  moved  along  the  shores  of  lakes  which  have  no  definite,  or  regular,  cur- 
rents. In  the  North  Atlantic  the  heavy  storms  are  northeasters  and  waves 
coming  from  this  direction  strike  a  blow  glancing  southward  along  the  coast; 
to  this  is  attributed  the  presence  of  rock  debris  in  the  beaches  far  southward 
from  its  place  of  origin. 

The  force  of  waves  may  be  very  great.  According  to  Stevenson  the  force 
with  which  the  average  waves  of  the  North  Atlantic  strike  in  summer  is 
about  600  pounds  per  square  foot;  in  winter  over  2000  pounds;  in  times  of 
severe  storms  over  6000  pounds.  Blocks  of  rock  10,  20,  and  50  tons  or  even 
more  in  weight  have  been  moved  by  heavy  breakers,  while  large  bowlders 
are  tumbled  in  the  surf  like  pebbles.  In  this  connection  it  should  be  recalled 
that  the  transporting  power  of  water  varies  as  the  sixth  power  of  the 
velocity,  as  demonstrated  under  rivers,  page  42.  These  facts  should  be 
borne  in  mind  when  erosion  by  waves  is  considered  later. 

• 
Destructive  Work  of  the  Ocean:  Erosion 

Ever  restless  is  the  sea ;  storms  arise  upon  it  and  the  storm  waves 
strike  upon  the  shore;  storms  pass  and  subside  but  the  surging 
ground-swell  continues  and  steadily  breaks  in  surf,  beating  on  the 
coast.  The  surface  of  the  ocean,  like  an  ever-moving  horizontal 
saw,  is  ceaselessly  cutting,  gnawing,  eroding  the  land.  This  it  does 
in  several  ways,  producing,  according  to  circumstances,  a  variety  of 
Matures  which  are  worthy  of  consideration. 


98  TEXT-BOOK    OF   GEOLOGY 

Wave  Erosion.  —  Sea-water  has  more  or  less  solvent  action  on 
various  kinds  of  rocks,  tending  to  disintegrate  them  and  thus  help- 
ing the  work  of  the  waves.  The  work  of  the  latter  is,  however, 
chiefly  mechanical.  Most  rock  masses  have  crevices  or  larger 
cracks  in  them,  and  the  air  or  water  in  these,  driven  violently  in  by 
the  impact  of  the  waves,  acts  as  a  wedge,  disrupting  them,  and  often 
dislodging  large  pieces.  In  this  way  heavy  masonry  is  often  torn 
asunder.  The  water,  rushing  into  cavities  and  suddenly  retreating, 
leaves  a  partial  vacuum  which  tends  to  suck  away  portions  of  the 
roof  and  sides,  and  the  constant  repetition  of  this  gives  rise  to  sea- 
caves,  blowing  holes,  and  spouting  rocks,  so  frequently  seen  on 
rocky  coasts.  But  the  chief  eroding  action  is  accomplished  by 
grinding,  and  in  performing  this  work  the  waves  use  as  tools  the 
dislodged  material,  and  that  which  falls  from  above  through  atmos- 


Land 


Water  level 


Fig.  74.  —  Diagram  illustrating  the  production  of  a  sea-cliff,  wave-cut  terrace  and 
the  wave-built  terrace. 

pheric  agencies,  and  which  would  naturally  form  a  talus  at  the  foot 
of  the  sea- cliff.  The  constant  striking  and  grinding,  not  only  of 
sand  and  gravel,  but,  as  mentioned  above,  even  of  heavy  bowlders 
render  the  waves  formidable  agents  of  destruction,  through  whose 
work  even  the  hardest  rocks  are  rapidly  worn  away. 

In  order  to  accomplish  this  task  waves  must  have  tools  to  work  with,  as 
stated  above.  Aside  from  the  mechanically  disrupting  process,  waves  of  pure 
water  have  little  eroding  power.  This  is  strikingly  shown  in  places  on  the 
coast  of  Norway,  where  headlands  are  brought  into  water  too  deep  to  be 
affected  by  coastal  debris.  The  rock  surfaces,  smoothed  and  furrowed  by 
former  action  of  glacial  ice,  still  retain  these  characteristic  features,  though 
subjected  to  the  constant  washing  of  the  waves. 

In  this  process  the  material  used  by  the  waves  is  itself  ground  up 
and  consumed  —  reduced  to  sand  and  silt.  It  loses  its  angular  char- 
acter and  becomes  rounded,  the  characteristic  form  of  coastal  debris 
submitted  to  chafing  by  the  waves,  and  similar  to  that  of  pebbles 
which  have  suffered  long  transport  in  rapid  streams. 

For  the  process  of  wave  erosion  to  continue,  the  ground-up  ma- 


THE  OCEAN  AND  ITS  WORK  99 

terial  must  be  removed,  like  sawdust  from  the  track  of  the  saw,  in 
order  that  fresh  rock-surfaces  may  be  exposed  to  attack,  otherwise 
the  fine  material  would  act  as  a  buffer  to  receive  the  blows  of  the 
waves,  and  would  prevent  further  erosion.  -This  removal  is  done  by 
the  undertow,  which  takes  the  debris  back  into  the  sea,  and  also 
by  tidal  and  littoral  currents,  which  carry  it  away.  What  they  do 
with  it  we  shall  presently  see.  Were  it  not  for  the  aid  of  these 
agencies  wave  erosion  would  cease,  except  where  the  sea  might  be 
invading  a  continually  sinking  land  surface. 

Sea-cliff  and  Terrace.  —  The  effective  erosion  of  waves  is  con- 
fined to  the  narrow  zone  within  which  they  work.     The  vertical 


Fig.  75.  —  Sea-cliff  and  terrace  extending  out  below,  cut  in  rock  by  the  waves.     Note 
absence  of  talus  at  foot  of  cliff.     Cape  Blomidon,  Bay  of  Fundy. 

height  is  of  course  increased  by  the  lift  given  to  the  surface  by  the 
tide.  This  perpendicular  distance  may  be  considered  the  width  of 
the  cut  made  by  the  horizontal  saw,  which  the  surface  of  the  sea 
forms.  Fig.  74  shows  this  cut  made  by  the  waves  into  the  land. 
As  this  process  goes  on,  by  undermining,  and  aided  by  the  action  of 
weathering,  material  is  dislodged  from  above,  falls,  and  is  ground  up, 
and  thus  the  sea-edge  is  terminated  by  a  cliff,  where  it  is  advancing 
inland  on  higher  country.  Beneath  the  surface  of  the  sea  at  the 
foot  of  the  cliff  lies  an  area  covered  with  shallow  water,  or  even 
partly  exposed  at  low  tide,  which  marks  the  lower  limit  of  wave 
action.  This  is  called  the  shore  platform,  or  wave-cut  terrace.  By 
the  action  of  sand  and  shingle,  swept  about  by  the  washing  of  the 
waves  and  the  undertow,  the  terrace  is  ground  away  downward  and 


100 


TEXT-BOOK    OF    GEOLOGY 


slowly  deepens  seaward  to  the  place  where  the  depth  prevents  such 
work  and  the  material  lies  at  rest.  Here  the  ground-up  material 
accumulates  and  this  deposit  is  known  as  the  wave-built  terrace. 
These  relations  are  shown  in  the  diagram  Fig.  74,  and  the  view  in 
Fig.  75  is  of  a  sea-cliff  and  a  wave-cut  terrace  in  rock  at  its  foot. 
See  also  Figs.  76  and  77. 

The  nature  of  the  sea-cliff  depends  very  much  on  the  character  of  the  ma- 
terial attacked  by  the  waves;  thus  in  hard  rocks  it  may  be  very  steep,  per- 


Fig.  76.  —  Irregular  coast-line  and  sea-cliff  produced  by  erosion  in  nearly  vertical  rock 
strata.     Pembroke,  Wales.     Geol.  Surv.  of  England  and  Wales. 

pendicular,  or  even  overhanging;  in  sand,  since  the  latter  would  be  under- 
going constant  undermining  and  sliding  down,  it  may  have  no  steeper  angle 
than  that  of  sand  at  rest.  It  also  depends  on  the  relative  rate  of  weathering 
as  compared  with  that  of  wave  cutting ;  thus  clay,  which  is  very  tenacious,  but 
easily  cut  by  the  waves,  may  present  bold  bluffs,  though  of  soft  material, 
whereas  granite,  which  is  very  hard,  and  attacked  and  worn  with  difficulty,  may, 
through  its  cracks  and  joints,  be  subect  to  more  rapid  disintegration  by  the 
action  of  frost  (see  page  21)  and  the  chemical  effect  of  salt  water,  and  thus 
present  a  sea-front  of  low  slope.  It  is  thus  not  so  much  the  actual  hardness 
of  the  material  as  its  relative  resistance  to  the  two  kinds  of  wear,  wave 
erosion  compared  to  weathering,  which  determines  the  nature  of  the  front 
which  the  coast  presents  to  the  sea.  What  was  said,  page  49,  regarding  the 
form  of  river  valleys  may  be  considered  in  this  connection.  From  what  has 
been  said  it  will  be  readily  understood  that,  unlike  cliffs  on  land  which 


THE  OCEAN  AND  ITS  WORK 


101 


accumulate  a  talus  below,  at  the  foot  of  a  sea-cliff  this  may  be  small  or 
wanting. 

Such  a  bench,  or  terrace,  cut  in  rock  and  terminated  inland  by  a  cliff,  and 
often  with  characteristically  rounded  pebbles  and  shingle  lying  upon  it  or 
heaped  at  the  foot  of  the  steep,  is  a  decisive  sign  of  surface  water-work, 
either  of  the  sea,  or  of  a  lake.  And  by  finding  these  inland,  removed  from 
the  present  water  edge  and  higher  than  its  level,  we  are  able  to  recognize  that 
a  change  of  the  water-level  has  taken  place;  either  that  the  land  has  been 
raised,  if  bordering  the  sea-shore,  or  that  the  water-surface  has  sunk,  if  about 
lakes  and  inland  seas.  Thus  the  terraces  about  Great  Salt  Lake,  far  up  on 


Fig.  77.  —  Sea-cliff  and  stack,  the  latter  a  remnant  of  the  former  land  now  eroded 
away.     Coast  of  Wales.     Geol.  Surv.  of  England  and  Wales. 

the  slopes  of  the  basin  and  above  the  present  lake,  which  are  shown  in  Fig. 
67,  prove  to  us  that  the  basin  was  once  filled  to  this  height.  This  series  of 
benches  show  the  successive  levels  of  the  lake  formed  in  the  process  of  its 
drying  up.  Many  other  similar  instances  of  wave-cut  terraces,  or  shorelines, 
elevated  above  present  water  levels,  could  be  mentioned,  such  as  those  about 
the  Baltic  Sea,  which  prove  elevation  of  the  land.  See  Fig.  182. 

Some  geologists,  more  especially  in  the  past,  have  believed  in  the  wide  ex- 
tension of  shore  platforms,  and  have  ascribed  large  areas  of  level  land,  com- 
posed of  worn-down  rocks,  to  long  and  vast  inroads  of  the  sea  and  to  subse- 
quent uplift;  these  have  been  called  plains  of  marine  denudation.  Later, 
without  denying  the  possibility  of  such  planing  off  of  the  land  by  the  ocean,  it 
was  held  more  probable  that  where  level,  or  nearly  level,  regions  had  been 
made  by  the  obvious  wearing  down  of  former  uneven  rock  surfaces,  this  had 
been  accomplished  by  atmospheric  erosion,  by  subaerial  peneplanation,  as 
described  on  page  70. 


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There  is  now  a  tendency  to  revert  to  the  former  idea,  and  to  recognize  the 
existence  of  marine  peneplains,  though  perhaps  not  of  such  wide  extent  as  was 
formerly  believed  in,  as  well  as  those  of  subaerial  origin.  Thus,  whether  a 
given  peneplain  has  been  made  by  stream  agencies,  or  by  the  sea,  must  be 
determined  in  each  case  by  the  particular  set  of  geologic  features  which  char- 
acterize it,  some  of  those  which  distinguish  marine  platforms  being  men- 
tioned above. 


Fig.  78.  —  Map  of  a  part  of  the  west  of  Norway,  showing  the  fiords  and  outer  islands 
of  a  submerged  coast-line. 

Irregularities  of  Coast-line.  —  The  sea,  advancing  upon  the  land, 
finds  materials  of  very  different  kinds  in  different  places  to  oppose 
its  progress,  and  thus  upon  the  nature  of  the  rocks  and  their  dis- 
position the  characters  of  the  coast-line,  especially  with  respect  to 
its  minor  features,  very  largely  depend.  See  Fig.  76. 


Thus  if  the  rocks  are  composed,  as  is  frequently  the  case,  of  parallel  con- 
cordant layers  or  beds,  called  strata,  and  their  edges  are  exposed  to  the 


THE  OCEAN  AND  ITS  WORK 


103 


waves,  the  weaker,  softer  layers  are  rapidly  worn  away  and  the  harder  beds, 
left  unsupported,  break  away  in  blocks.  If  the  beds  are  horizontal,  the 
harder  layers  may  project  for  a  time  as  table  rocks  with  cavities  under  them, 
as  on  the  coast  of  Lake  Superior.  If  the  beds  are  vertical,  or  inclined,  but 
with  edges  exposed,  the  hard  layers  stand  out  like  columns  or  ribs.  If  the 
face  of  the  beds  is  towards  the  sea,  erosion  is  slower  because  the  hard  layers 
form  an  apron,  or  wall,  to  protect  the  soft  layers  behind  them.  If  the  rock 
masses  are  homogeneous  and  hard,  like  trap  or  granite,  the  irregu- 
larities are  largely  determined  by  the  joints,  or  regular  system  of  cracks  in 
them,  and  how  these  are  disposed  toward  the  sea  front.  Finally,  if  along 
the  coast  there  are  here  and  there  hard  masses  with  softer  ones  intervening, 


•* 


Fig.  79.  —  The  Sogne  fiord,  from  Gudvangen,  Norway. 

the  latter  are  worn  away  and  make  coves,  while  the  more  resistant  masses  pro- 
ject as  headlands.  Thus  a  bold  coast  facing  the  sea  is  liable  to  show  many 
minor  irregularities  of  topography. 

The  sea  in  its  production  of  an  irregular  shore-line  by  dissection 
of  the  coast,  may  cut  off  portions  of  land  and  turn  them  into  islands. 
Or  the  portions  thus  isolated  may  be  bold  masses  of  rock  of  varied 
form  and  appearance,  scarcely  large  enough  or  far  enough  from  the 
sea-cliffs  to  be  dignified  as  islands,  which  are  known  as  stacks  or 
chimney  rocks.  They  are  illustrated  in  Fig.  77  and  are  common 
features  in  various  places,  as  on  the  coasts  of  Great  Britain. 

Indented  Coast-lines;  Fiords  and  Estuaries.  —  The  shores  of 
many  countries,  of  which  the  coasts  of  eastern  North  America,  of 
Norway,  of  Alaska,  and  of  southern  Chile  may  be  cited  as  examples, 


104  TEXT-BOOK   OF    GEOLOGY 

present  deeply  indented  outlines;  usually  there  is  an  outer  fringe 
of  islands  and  behind  this  are  many  long  bays  retreating  inland 
and  forming  fiords  into  whose  heads  the  rivers  empty.  This  is 
illustrated  in  the  map  of  a  portion  of  the  coast  of  Norway,  Fig.  78, 
where  these  fiords  wind  back  into  the  country  for  great  distances, 
100  miles  or  even  more  in  some  cases,  are  very  narrow  and  deep, 
and  often  bordered  by  precipitous  walls  from  1000  to  3000  feet 
high.  A  view  of  one  is  seen  in  Fig.  79.  Somewhat  similar  fiords  are 
found  in  Alaska,  Chile,  and  other  places. 

It  is  impossible  to  believe  that  coasts  with  outlines  indented 
deeply  on  such  a  scale  by  embayments  having  high  rocky  walls, 


Fig.  80.  —  Former  drowned  valley  forming  an  estuary,  now  filled  by  deposits  and 
changed  to  a  tidal  marsh.     Cohasset,  Mass. 

down  which  cataracts  descend  from  hanging  valleys  above,  as 
shown  in  Fig.  79,  could  have  been  made  solely  by  the  action  of  the 
waves,  or  by  the  drowning  of  normal  river  valleys  through  sub- 
mergence. The  most  satisfactory  explanation  for  them  seems  to  be 
that  in  a  former  time  those  great  streams  pf  ice,  called  glaciers, 
filled  what  were  once  river  valleys  and  eroded  them  below  sea-level : 
after  this  the  ice  melted  and  the  sea  water  came  in  to  drown  these 
over-deepened  valleys.  Fiords,  therefore,  are  glacial  troughs,  whose 
lower  ends  have  been  flooded  and  turned  into  arms  of  the  sea.  The 
manner  in  which  glaciers  perform  this  work  will  be  described  in  the 
following  chapter. 

On  the  other  hand,  in  distinction  from  such  fiord  shore-lines  there 
are  many  deeply  indented  coasts  representing  tracts  of  country  with 
very  irregular  topography,  which,  by  gradual  sinking  of  the  land, 
have,  in  part,  become  lower  than  sea-level,  and  whose  river  valleys 


THE  OCEAN  AND  ITS  WORK  105 

have  therefore  been  submerged,  or  drowned.  Such  drowned  valleys, 
formed  normally  by  rivers,  are  called  estuaries.  Delaware  and 
Chesapeake  bays  and  Long  Island  Sound  are  examples  of  such  wide 
and  open  valleys  which  have  been  submerged.  That  the  estuaries 
along  the  Atlantic  coast  have  been  made  in  this  way  is  further 
proved  by  the  fact  that  the  extensions  of  the  present  river  channels, 
or  valleys  made  when  the  land  stood  emerged,  have  been  traced  by 
soundings  as  deep  furrows  across  the  continental  shelf.  The  drown- 
ing of  river  valleys  by  submergence  has  been  previously  mentioned 
on  page  72.  Indented  coasts  of  this  nature  are  said  to  have  ria 
shore-lines,  after  the  term  applied  to  the  northwest  coast  of  Spain, 
which  is  cited  as  the  classic  example. 

The  outer  fringe  of  islands,  often  seen  along  such  coasts,  are  the  tops  of 
hills  and  more  elevated  tracts,  nearer  the  former  shore,  which  by  submergence 
have  been  cut  off  from  the  mainland.  It  should  also  be  remembered  that, 

hile  the  larger  features  of  such  shore-lines  are  due  to  submergence,  subse- 

uent  wave  work  and  erosion  may  have  done  much  to  modify  them  and  to 

ive  them  their  present  aspect. 
Such  estuaries  are  gradually  being  filled  by  the  streams  which  empty  into 

.em.  Their  deposits,  which  may  be  considered  as  the  equivalents  of  the 
stream  deltas,  are  found  at  the  heads  of  the  estuaries  forming  level  stretches  of 
salt-water  marshes,  or  tidal  flats,  as  illustrated  in  Fig.  80,  now  flooded  by  fresh 
water  from  the  river,  now  covered  by  salt  water  from  high  tide.  The  extent 
to  which  these  tidal  marshes  have  advanced  and  filled  an  estuary  depends  on 
its  size  and  depth,  and  on  the  volume  of  material  furnished  by  the  inflowing 
streams.  See  also  drowned  rivers,  page  72.  Such  estuaries  from  the  in-  and 
outflow  of  the  tide  are  subject  to  strong  tidal  currents,  which  carry  away  in 
part  the  sediment  brought  in  by  the  rivers  and  thus  make  less  rapid  the 
progress  of  marsh  extension  at  its  head.  The  material  thus  swept  up  and 
down  the  estuary  is  often  partly  deposited  in  quiet  nooks  and  corners  to  form 
tidal  flats  and  marshes  and  thus  helps  in  such  places  in  estuary  filling.  It 
may  be  added  to  by  that  resulting  from  wave  erosion.  Another  portion  is, 
however,  swept  out  to  sea  by  the  out-going  tide,  and  aids  in  the  general  con- 
structive work  of  the  waves  and  currents  along  the  shore. 

Constructive  Work  of  the  Ocean:  Deposition 

We  have  already  seen  that  from  a  geological  point  of  view  a  lake 
must  be  regarded  as  a  temporary  affair;  however  large  it  may  be; 
in  the  vast  lapses  of  geologic  time  it  will  become  filled  up  with 
sediment  and  obliterated,  if  its  life  is  not  previously  cut  short  by 
draining.  Although,  as  previously  described,  an  enormous  quantity 
of  material  is  poured  into  the  sea  by  the  rivers  from  the  erosion  and 
aste  of  the  land,  and  this  is  aided  by  the  attack  of  the  waves  on 
e  coast,  the  ocean  basins  are  too  enormous  to  be  filled  by  such 


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TEXT-BOOK   OF    GEOLOGY 


means,  for  if  all  of  the  land  were  reduced  to  sea-level  and  the  ma- 
terial spread  over  the  ocean  floor  the  average  depth  would  only  be 
reduced  about  700  feet.  In  other  words  the  oceanic  level  would 
everywhere  be  raised  less  than  700  feet. 

But  as  the  material  resulting  from  the  wear  of  the  land  is  mostly 
deposited  in  shallow  water,  in  a  relatively  narrow  zone  on  the  con- 
tinental shelves,  by  its  concentration  it  becomes  a  matter  of  geologi- 
cal importance  and,  since  much  of  it  lies  within  reach  of  the  action 
of  waves  and  currents,  it  is  used  by  them  in  the  construction  of  new 
shore  features  which  are  of  interest  to  consider.  But  sediments  of 
this  nature,  along  the  shores  on  the  continental  shelves,  are  by  no 
means  the  only  ones  occurring  on  the  ocean  floor,  for  even  over  the 
bottom  of  its  deep  basins,  and  produced  by  several  agencies,  de- 
posits are  taking  place.  We  may  thus  classify  under  two  heads  the 
oceanic  sediments,  shallow-water  deposits  and  deep-sea  deposits. 
We  will  first  consider  the  former. 

Beach.  —  This  is  the  familiar  feature  which  distinguishes  the 
edge  of  the  sea,  or  lakes.  It  consists  of  the  material  which  is  being 
worked  over  and  ground  up  by  the  waves.  Its  upper  edge  is  often 
marked  by  a  belt  of  coarser  material  thrown  up  and  left  by  the 

heaviest  waves.  Lower 
down  it  consists  of  the 
finer  sand  and  shingle 
swept  in  and  out  and 
spread  by  the  waves,  un- 
dertow, and  littoral  cur- 


Water  level 


Fig.  81.  —  Section  of  a  beach,  after  Gilbert. 


rents.  See  Fig.  81.  Often  beaches  are  inconspicuous  or  wanting 
at  the  foot  of  steep  sea- cliffs  and  on  the  contrary  are  finely  dis- 
played at  the  end  of  coves  and  bays,  often  in  curving  outline,  the 
reason  being  that  the  constant  agitation  of  the  waves  at  the  ex- 
posed headlands  keeps  the  ground-up  material  in  suspension,  and 
the  tidal  and  littoral  currents  carry  it  away  along  shore  until  it  is 
deposited  in  these  quieter  and  more  sheltered  places.  This  is 
especially  true  in  lakes.  See  page  97.  A  view  of  a  beach  thus 
formed  is  shown  in  Fig.  82. 

It  is  evident  that,  as  the  ocean  advances  on  the  land  by  its  erosive  work, 
its  edge  is  marked  by  the  constantly  progressing  beach.  If  this  should  take 
place  on  the  side  of  a  continental  mass  which  is  sinking,  the  advance  of 
the  beach  inland  becomes  more  rapid.  In  a  subsiding  land  area,  then,  every 
part  is  first  swept  over  by  a  beach  line  before  it  becomes  the  ocean  bot- 
tom. The  consequences  of  this  are  marked  and  of  great  interest  as  will  be 
developed  later.  Beaches  have  certain  marked  characters  which  are  de- 
scribed under  stratification  and  by  means  of  them  we  are  able  to  recog- 


THE  OCEAN  AND  ITS  WORK 


107 


nize  the  fact  that  what  are  now  land  surfaces  have  been  many  times  sub- 
merged beneath  the  sea  and  again  raised. 

The  sands  spread  out  on  the  beach  become  dry  on  the  retreat  of  the 
tide  and  waves.  They  are  then  subject  to  the  action  of  the  atmosphere. 
Since  along  coasts  the  strongest  winds  are  apt  to  come  from  the  sea,  the 
sand  is  lifted  and  dropped  inland,  forming  sand-dunes,  as  described  on 
page  14.  Hence,  on  low  coasts,  lines  of  sand-dunes  back  of  the  beach  are 
a  very  common  feature. 


Fig.  82.  —  A  curving  beach.     Conception  Bay,  Newfoundland.     C.  D.  Walcott, 

U.  S.  Geol.  Surv. 

Barriers.  —  If  the  part  of  the  ocean  bottom  forming  the  con- 
tinental shelf  slopes  out,  seaward,  very  gradually,  then  the  de- 
posits brought  down  by  the  rivers  and  the  products  of  wave 
erosion,  which  are  spread  about  on  it  by  the  tidal  and  littoral 
currents,  may  be  exposed  to  the  action  of  waves  in  shallow 


Land 


Barrier 


Sea-level 


Fig.  83.  —  Formation  of  a  barrier  sand  reef. 

water  at  a  considerable  distance  from  the  edge  of  the  land.  When 
the  waves  moving  landward  begin  to  drag  on  the  bottom,  the 
tops,  moving  faster,  curl  over  and  breakers  are  formed;  the  mere 
wave-form  here  changes  to  one  of  actual  onward  movement  of 
the  water,  which  rushes  forward,  tearing  up  the  sand  and  dragging 
it  along.  The  undertow  sweeps  the  sand  and  shingle  back,  and 
the  material  thus  put  in  motion  is  heaped  up  at  the  point  off 
hore  where  the  struggle  between  the  land  and  sea  begins,  forming 


108 


TEXT-BOOK    OF   GEOLOGY 


a  long  narrow  bar,  or  barrier,  parallel  to  the  general  trend  of  the 
coast,  as  illustrated  in  Fig.  83.  The  waves  beating  on  this  may 
throw  up  the  sand  until  it  rises  to  the  surface;  if  broad  enough 
the  winds  may  continue  the  work,  lifting  the  sand  into  dunes, 
and  thus  a  long,  low,  and  narrow  island  fronting  the  mainland 
may  be  developed.  Between  it  and  the  mainland  lies  a  stretch 
of  shallow  water,  rarely  more  than  20  feet  deep,  called  a  lagoon  if 
small,  or  a  sound  if  large.  These  are  illustrated  in  Fig.  84. 

It  is  evident  that  the  distance  of  such  a  barrier  from  the  mainland  will 
depend  on  the  seaward  slope  of  the  bottom.  Off  the  coast  of  North  Caro- 
lina, where  the  bottom  shelves  out  very  gradually,  they  are  far  out  with 
wide  sounds  behind  them,  as  seen  in  Fig.  84;  where  the  shore  slopes 

sharply  off  into  deep  water  they  are 

wanting,  as  along  the  coast  of  Cali- 
fornia. In  Florida  the  barrier  is  close 
in,  forming  a  long,  narrow  sound 
known  as  Indian  River. 

Such  a  barrier  may  be  built  across 
the  mouth  of  a  river,  forcing  it  to 
flow  parallel  to  the  coast  for  a  long 
distance,  or,  if  the  stream  is  feeble, 
completely  closing  the  estuary  and 
converting  it  into  a  fresh-water 
lake,  Fig.  87,  though  this  is  more 
commonly  done  by  bars,  as  de- 
scribed in  the  following  section.  Or- 
dinarily the  sweep  of  the  tide,  in 
and  out  of  such  sounds,  keeps,  by  its 
scour,  channel-ways,  called  inlets, 
open  to  the  ocean. 

These  bodies  of  water,  being  shal- 
low, soon  become  rilled  up  by  the 
sediments  brought  into  them  by  the 
streams,  the  tide,  and  by  accumula- 
tions of  animal  life,  such  as  shells, 
etc.,  and  of  vegetable  matter,  such 
as  peat.  Tfyus  they  may  become 
converted  into  tidal  flats  and  brack- 
ish or  salt-water  marshes,  and  these  by  further  growth  of  vegetation  and  up- 
building, or  by  draining  through  the  agency  of  man,  into,  eventually,  tillable 
lands.  The  student  will  also  notice  that  through  the  formation  of  barrier 
islands  a  much  indented  coast-line  may  be  simplified  on  the  ocean  front. 

Bars  and  Spits.  —  We  have  already  seen,  page  57,  that  when 
a  river  enters  the  sea,  or  a  lake,  the  sediments  it  deposits  form 
bars  at  its  mouth.  This  is,  however,  not  the  only  way  in  which  bars 
are  made,  for  they  are  formed  especially  by  waves,  and  by  littoral 


Fig.  84.  —  Map  of  North  Carolina  coast, 
illustrating  the  barrier  beach  and  en- 
closed sounds. 


THE  OCEAN  AND  ITS  WORK 


109 


and  tidal  currents  as  well,  and  those  at  the  mouths  of  rivers  may  be 
much  modified  as  to  their  disposition  by  waves  and  currents.  A 
current  moving  along  shore  may  carry  sediment  in  suspension, 
especially  if  the  water  is  agitated  by  storm  winds  (and  this  applies 
particularly  to  lakes  where  shore  currents  are  feeble) ;  if  it  now 
comes  to  a  narrow  indentation  of  the  coast  it  tends  to  move  across 
its  mouth,  rather  than  to  follow  its  shore,  and  coming  into  deeper 
water  it  is  slowed  and  the  sediment  deposited.  Thus  the  current 
tends  to  build  up  an  embankment  across  the  indentation  and  to 
form  a  bar.  The  necessary  conditions  are  that  the  current  should 


Fig.  85.  —  View  of  a  spit.     Duck  Point,  Grand  Traverse  Bay,  Lake  Michigan.     I.  C. 
Russell,  U.  S.  Geol.  Surv. 

be  fed  with  sediment  from  the  beach  along  which  it  moves,  and 
that  it  should  proceed  into  deeper  water  where  it  slows  up  and 
deposits  its  load.  When  the  current  has  built  the  deposit  up  to  the 
point  where  the  waves  may  begin  work  upon  it,  these  may  still 
further  heap  up  the  material  until  it  becomes  land,  and  the  winds,  by 
raising  dunes  upon  it,  may  increase  its  height  still  more.  If  the  em- 
bankment projects  out  with  a  free  end  it  is  called  a  spit,  but  if  it 
closes,  or  nearly  closes,  the  indentation  of  the  coast  it  is  a  bar.  See 
Figs.  85  and  86.  Commonly  the  scour  of  tidal  currents,  rushing  in 
and  out  of  the  bay,  prevents  complete  closure  and  leaves  an  inlet; 
but  closure  sometimes  occurs,  and  the  enclosed  water-body,  after  it 
las  been  rinsed  out  by  the  drainage  passing  into  it,  becomes  a 
fresh-water  lake.  Thus  by  the  formation  of  bars  and  barriers,  or 
)mbinations  of  them,  numerous  small  lakes  and  ponds  along  the 


110  TEXT-BOOK    OF   GEOLOGY 

'coasts  have  been  made.  Many  excellent  examples  are  found  in 
New  England,  as  illustrated  in  Fig.  87.  Spaces  between  islands,  or 
between  an  island  and  the  mainland,  are  often  favorable  places  for 
the  formation  of  a  bar,  and  it  is  common  to  find  islands  tied  to- 
gether, or  to  the  mainland,  by  them,  as  illustrated  in  Fig.  86.  They 
occur  also  in  lakes. 

Bars  may  also  be  formed  in  shallow  seas  or  on  the  continental  platforms  at 
considerable  distances  from  land  by  the  action  of  tidal  currents.  Thus  the 
material  brought  down  by  the  rivers  of  southern  England  and  northern 
France  into  the  British  Channel,  and  that  worn  from  their  coasts  by  wave 


I 


Fig.  86.  —  Island  tied  to  mainland  by  bar.     Bay  of  Fundy,  Nova  Scotia. 


erosion,  is  in  large  part  swept  by  the  tidal  currents  into  the  North  Sea,  where 
the  currents  meeting  the  advance  of  the  tide  coming  into  this  sea  from  its 
northern  opening  come  to  rest  at  high  tide,  and  deposit  their  load  of  sedi- 
ments, forming  the  numerous  shoals  and  bars  which  characterize  its  bot- 
tom. The  chief  features  of  the  relief  of  the  bottom  of  this  sea,  how- 
ever, are  probably  due  to  the  irregularities  of  an  old  land  surface,  recently 
depressed  below  sea-level. 

Deep-water  Deposits.  —  The  deposits  which  have  been  described 
in  the  foregoing  sections,  and  which  are  used  by  the  ocean  in  its  con- 
structive work  of  making  beaches,  barriers,  bars,  and  islands,  are 
very  largely  those  occurring  in  very  shallow  water  close  to  the  land, 
where  the  greater  part  of  the  land  waste  is  laid  down.  But  in  ad- 
dition to  these,  as  was  intimated  in  a  previous  section,  deposits  are 
laid  down  in  deep  as  well  as  in  shallow  water,  and  indeed  one  may 


THE  OCEAN  AND  ITS  WORK 


111 


say  that,  of  one  kind  or  another,  they  are  formed  everywhere  on 
the  floor  of  the  sea.  Since  in  several  ways  they  are  both  important 
and  of  interest,  they  deserve  consideration. 

Going  seaward  from  the  land  we  may,  for  practical  purposes, 
divide  the  ocean  bottom  into  three  zones  or  regions,  the  continental 
shelves,  the  intermediate  slopes,  and  the  profound  abyss,  as  illus- 
trated in  the  diagram,  Fig.  88,  each  distinguished  by  certain  charac- 
ters as  well  as,  in  a  general  way,  by  the  deposits  occurring  on  them, 
as  follows: 

The  Continental  Shelf.  —  This  has  been  already  described.  In 
general  it  is  limited  seaward  by  the  depth  of  100  fathoms  (600  feet). 


POINT  JUDITH 


'WATCH  HILL   PT. 


Fig.  87.  —  Map  of  coastal  fresh-water  ponds.     Rhode  Island. 

Taking  the  whole  world  into  account  the  area  of  sea-bottom  belong- 
ing in  this  zone  is  about  10,000,000  square  miles.  It  can  be  sub- 
divided into  the  littoral  or  beach  region,  between  high-  and  low- 
water  marks,  and  the  shallow-water  area,  beyond  low-water  mark, 
and  therefore  never  exposed  to  the  air.  The  littoral  zone  covers  a 
relatively  small  space,  estimated  at  about  62,500  square  miles  for  the 
world. 

Epeiric  Seas.  —  The  shallow-water  area,  in  addition  to  the 
epi continental  seas  covering  the  continental  shelves,  as  previously 
described,  includes  basins  more  or  less  enclosed  by  land,  which  the 
overflooding  of  the  ocean  has  filled  with  salt  water.  Seas  enclosed 
by  land  may  be  divided  into  two  classes,  as  follows:  first,  those 
which  are  very  deep,  and  through  geologic  periods,  without  regard 
to  relative  changes  of  level  in  land  and  sea,  have  maintained  them- 
selves as  water-bodies;  and  second,  those  formed  in  depressed 
tracts  or  down-warps  of  the  continental  masses,  which  have  in  times 


112 


TEXT-BOOK    OF    GEOLOGY 


past  experienced  great  changes  through  variations  of  sea-level, 
sometimes  being  more  or  less  completely  emptied  of  their  water,  or 
filled  with  sediments  and  turned  into  land.  Examples  of  the  former 
class  are  to  be  seen  in  the  Mediterranean  and  Caribbean  seas,  which 
are  very  deep;  these,  however,  are  not  true  seas  but  are  known 
as  mediterraneans.  Hudson  Bay,  the  Gulf  of  St.  Lawrence,  and 
the  Baltic  Sea  are  existing  examples  of  the  second  class.  Such 
shallow  seas  as  these  latter,  from  their  relations  to  the  continents, 
may  be  termed  epeiric  seas  (Greek,  ^Treipos,  a  continent),  and  those 
of  North  America,  as  we  shall  see  in  the  second  part  of  this  work, 
have  been  in  the  past  the  theater  of  important  events. 

Characters  of  Shallow  Waters.  —  In  the  areas  of  shallow  water 
described,  the  epicontinental  and  epeiric  seas,  the  following  char- 
acters obtain:  The  waters  are  more  or  less  agitated  to  the 


Sea  level 


Fig.  88.  —  Diagram  showing  different  ocean  zones. 

bottom  by  wave  movements,  and  are  kept  in  motion  by  tidal 
and  ocean  currents.  They  are  influenced  by  external  temperatures, 
and  thus  experience  seasonal  changes  from  warmer  to  colder  and 
the  reverse.  Over  the  floor  of  the  continental  shelves  and  inland 
basins  the  deposits  are  chiefly  those  sediments  coming  from  the  land, 
terrigenous  (born  of  the  land)  as  they  are  called,  mainly  sands  and 
muds  and  their  intermixtures.  In  places,  where  land  sediments 
are  scanty,  or  wanting,  as  in  shallow-water  districts  about  the  south 
coast  of  Florida,  or  in  the  open  ocean,  the  sediments  may  be  wholly 
produced  by  organic  life,  as  described  later  under  that  agency,  and 
are  then  mainly  composed  of  carbonate  of  lime,  such  as  micro- 
organisms, shells  of  various  kinds,  corals,  etc. 

In  this  zone,  owing  to  the  conditions  described  and  to  the  fact  that  light 
penetrates  freely,  various  forms  of  life  are  abundant  on  the  bottom.  Vege- 
tation, such  as  seaweeds  (algae),  flourishes  and  upon  it  are  nourished  different 
kinds  of  vegetable-eating  animal  organisms  such  as  certain  snail-like  shell- 
fish, herbivorous  gastropods,  and  worms  for  example,  and  upon  these  live 
carnivorous,  or  flesh-eating,  animals  such  as  certain  kinds  of  fish.  A  large 
part  of  the  commercial  food-fishes  live  in  this  region.  The  plants  and  ani- 
mals of  the  shelves,  owing  to  the  presence  of  light,  exhibit  colors,  of  different 
tints,  which  are  often  remarkably  brilliant  and  varied.  There  is  also  in  the 


THE  OCEAN  AND  ITS  WORK  113 

sea  an  immense  quantity  of  minute  floating  forms  of  vegetation  (algae),  and 
upon  these  microscopic  plants  various  animal  organisms  subsist,  to  be  them- 
selves devoured  in  turn.  This  floating  life  is  a  great  magazine  of  food. 

The  abundance  of  life  in  the  sea  varies  much  between  cold  and  warm 
waters,  and  also  depends  on  food  supply  and  oxygen.  Of  the  chief 
foodstuffs  there  is  usually  an  ample  quantity;  the  determining  factor  appears 
to  be  the  amount  of  certain  substances  needed  by  life  in  small  quantities, 
which  are  brought  from  the  land  into  the  sea  by  rivers.  Nitrogen  in  some 
form  of  combination,  phosphorus,  and  silica,  are  examples  of  this.  It  is  thus 
easy  to  see  why  life  is  more  abundant  around  sea-coasts  than  out  in  the  open 
ocean.  Furthermore,  in  the  shallow  coastal  and  epeiric  seas  of  tropical  regions, 
it  has  been  found  that  certain  bacteria  swarm,  which  have  the  property  of 
secreting  and  precipitating  lime  from  sea-water  and  at  the  same  time  of  con- 
verting the  combined  nitrogen  into  nitrogen  gas.  But,  as  is  well  known, 
warm  waters  can  contain  less  gas  in  solution  than  cold  ones ;  hence  the  nitrogen 
tends  to  escape  and  the  water  to  absorb  a  relatively  lesser  amount  of  oxygen 
from  the  air.  Thus  in  great  measure  such  seas,  deprived  in  considerable  part 
of  the  necessary  staple  of  life,  combined  nitrogen,  and  with  lower  content  of 
oxygen,  are  not  favorable  for  the  production  of  life,  first,  vegetable,  and 
second,  animal,  which  subsists  upon  vegetable  life,  in  great  quantities. 

In  such  seas  in  temperate  or  arctic  climates,  on  the  other  hand,  the  waters 
are  colder,  the  denitrifying  bacteria  do  not  exist,  or  are  present  in  small  num- 
bers, there  is  more  oxygen  in  solution,  the  conditions  for  life  are  better,  and 
hence  we  find  their  shores  and  bottoms  thickly  populated  with  an  abundance 
of  organisms,  both  animal  and  vegetable.  It  is  in  such  waters  that  the  great 
fisheries  of  the  world  are  situated.  The  popular  notion  that  tropical  waters 
must  swarm  with  life  because  they  are  warm  is  quite  incorrect,  but,  on  the 
other  hand,  it  is  true  that  the  variety  of  living  forms  is  far  greater  in  warm 
waters  than  in  cold  ones,  and  the  amount  of  carbonate  of  lime  deposited  by 
them  is  much  larger. 

The  deposits  as  laid  down  are  in  concordant  layers,  or  stratified,  and  exhibit 
certain  characters  described  later  in  detail  under  stratification;  they  may 
contain  in  great  abundance  remains  of  animals,  such  as  shells,  etc.,  and  even  of 
plants,  drifted  into  them  from  the  land.  Thus  this  region  and  its  deposits  are 
of  great  interest,  on  account  of  its  forms  of  life,  not  only  to  the  zoologist  and 
botanist,  but  to  the  geologist  as  well,  since  by  study  of  them  he  is  able  to  per- 
ceive that  wide  stretches  of  land,  now  covered  with  stratified  beds  of  rock 
and  full  of  organic  remains  (fossils),  were  once  areas  of  shallow  sea-bottom 
which  have  become  dry  land,  and  thus  exposed,  and  to  understand  the  condi- 
tions under  which  the  bedded  rocks  were  laid  down  and  the  animal  life 
flourished,  as  explained  later  in  this  volume. 

The  Intermediate  Slope.  —  This  constitutes  that  portion  of  the 
ocean  bottom  out  beyond  the  100  fathom  (600  feet)  line  to  a  depth 
of  1000  fathoms  (6000  feet) .  Although  the  upper  border,  which  is 
the  edge  of  the  continental  shelf,  is  quite  well  defined,  the  lower 
limit  is  largely  one  of  convenience.  It  may  be  considered  the  wall 
or  side  of  the  true  ocean  basins,  as  illustrated  in  Fig.  88,  but  it  must 
not  be  understood  that  there  is  always,  and  everywhere,  a  marked 


TEXT-BOOK   OF   GEOLOGY 

change  of  slope,  and  a  very  sharp  descent  from  the  edge  of  the  conti- 
nental shelf  to  the  profound  abyss,  for,  although  this  is  relatively 
true  in  a  general  way,  the  slope  outward  is  often  a  gradual  one,  pro- 
longed for  great  distances. 

In  this  region,  which  covers  about  18,000,000  square  miles  of  the 
ocean  bottom,  the  lowest  layers  of  water  are  not  agitated  by  waves, 
but  only  by  ocean  currents  of  slow  movement.  The  temperature  is, 
in  general,  fairly  constant  and  not  influenced  by  seasonal  changes  of 
heat  and  cold.  However,  when  warm  and  cold  currents  pass  near 
one  another  there  are  at  times  sudden  changes  and  destruction  of 
life.  Light  is  absent,  or  only  very  feeble  near  the  edges  of  the  con- 
tinental shelf,  having  been  absorbed  before  penetrating  such  depths. 
The  deposits  are  only  the  very  finest  of  the  land  sediments  which 
have  been  drifted  out.  This  is  especially  the  situation  of  the  blue 
and  green  muds.  The  voyage  of  the  Challenger,  an  exploring  vessel 
sent  out  by  the  British  Government,  showed  that  even  at  distances 
of  from  150  to  200  miles  the  approach  to  land  could  be  told  by  these 
muds. 

The  lower  limit  of  the  muds  is  indefinite  and  often  extends  far 
out  beyond  the  6000  feet  line.  The  blue  muds  have  been  estimated 
to  cover  14,000,000  square  miles  of  bottom.  In  places  they  are  re- 
placed by  red  muds,  as  on  the  east  coast  of  South  America,  which 
should  not  be  confounded  with  red  clays  found  in  the  abysses,  and 
other  areas  of  the  slopes  may  be  in  part  covered  by  grey  muds 
composed  of  volcanic  ashes,  or  by  deposits  from  organic  life,  as 
mentioned  in  the  following  paragraph: 

Owing  to  the  absence  of  light  in  this  region  there  is  no  vegetable  life  on  the 
bottom,  or  but  a  small  amount  restricted  to  the  zone  near  the  continental 
shelf.  The  animal  life  is  mostly  confined  to  certain  forms  which  live  upon 
organic  matter  in  the  mud,  resulting  from  decay  of  the  sunken  masses  of 
floating  microscopic  plants  and  the  bodies  of  free-swimming  animals,  such  as 
crustaceans,  squids,  and  fishes,  which  inhabit  the  top  layers  of  water,  and  of 
vegetable  matter  carried  out  from  land  by  ocean  currents.  From  the  small 
amount  of  light,  or  its  absence,  the  living  forms  are  generally  of  dull  colors, 
yellows  and  browns  predominating.  The  green  color  of  these  muds  is  given 
to  them  by  grains  of  a  green  substance  called  glauconite,  a  silicate  of  alumina, 
iron,  and  potash,  which  forms  in  the  sea.  It  is  also  found  in  some  stratified 
rocks  formed  in  shallow  water,  and  thus  helps  to  prove  that  they  were 
former  sea-bottom  deposits. 

The  Profound  Abyss.  —  This  comprises  the  whole  area  of  the 
ocean  bottom  below  the  1000  fathom  (6000  feet)  mark.  In  it  are 
also  included  the  lower  parts  of  a  few  interior  basins,  such  as  the 


THE  OCEAN  AND  ITS  WORK 


115 


Fig.  89.  —  Deep-sea  calcareous  ooze,  much 
magnified,  containing  shells  of  Foraminifera. 
Agassiz  and  Murray. 


Black  Sea  and  the  Roman  Mediterranean.  The  deeps  attain  a 
maximum  of  31,614  feet.  In  these  profound  and  monotonous 
abysses  light  is  absent,  there 
is  no  movement  of  the  waters 
except  that  slow  transfer  re- 
sulting from  the  unequal  heat- 
ing of  the  surface  of  the  globe, 
and  the  movement  of  currents 
on  the  surface  due  to  the 
winds,  which  carries  the  cold 
water  of  the  Polar  regions 
into  the  depths  of  equatorial 
basins.  The  temperature  of 
bottom  layers  of  sea- 
is  that  at  which  it  has 

maximum  density,  about  34°  or  35°  Fahr.,  or  very  near  freezing. 
Over  by  far  the  greater  part  of  the  floor  of  these  abysses  land- 
lerived  sediments  are  wanting.     Over  all  of  them,  however,  by 
mnding,  peculiar,  fine,  soft  deposits,  called  oozes,  have  been  found, 
rhich  in  different  regions  have  different  characters.    These  are  so 
)ft  and  fine  that  water  movements,  of  one-half  mile  per  day  are 
stated  to  shift  them  on  the  bottom,  and  it  is  suggested  that  the 
lonotonous  character  of  the  sea-floor  may,  in  part,  be  due  to  the 
tiling  of  the  smaller  depressions  by  these  deposits.    There  are  three 
'•ays  in  which  these  oozes  can  originate:  a,  volcanic,  fine  dust  from 
volcanic  eruptions  carried  vast  distances  by  air  currents,  or  volcanic 
ishes  and  pumice  floating  and  driven  by  ocean  currents  long  periods 
jfore  sinking;  b,  cosmic,  the  particles  of  interstellar  space  which 
earth  gathers  in  its  journey  around  the  sun;  and  c,  organic,  re- 
ilting  from  the  shells  and  framework  of  organisms  living  in  the 
irface  layers  of  the  ocean.    Of  these  oozes  the  most  abundant  are  a 
peculiar  red  clay,  which  covers  the  floor  of  the  deepest  abysses  and 
has  been  estimated  to  have  an  extent  of  over  50,000,000  square 
miles,  chiefly  in  the  Pacific,  and  a  calcareous  ooze,  resulting  from  de- 
position of  the  shells  of  minute  organisms  and  estimated  to  cover 
nearly    as    large    an    area,    especially   in   the   Atlantic,    Fig.    89. 

In  the  Polar  oceans  a  siliceous  ooze  occurs,  formed  from  the  very  ornate 
shells  of  diatoms,  very  minute,  simple  floating  plants  whose  shell  is  composed 
of  silica  (SiO2).  As  the  calcareous  oozes  do  not,  in  general,  occur  in  the  greatest 
depths,  and  yet  life  is  nearly  everywhere  found  in  the  top  layers  of  the  oceans, 
it  is  inferred  that  the  calcareous  shells  of  the  organisms  are  dissolved  before 
hing  bottom  and  therefore  the  oozes  they  might  make  are  replaced  by  the 


reac 


116  TEXT-BOOK   OF   GEOLOGY 

red  clay  which  forms  as  a  final  residuum,  chiefly  from  the  decay  of  volcanic 
and  cosmic  materials.  The  presence  of  the  latter  is  shown  by  the  minute 
balls,  obtained  in  soundings,  which  contain  metallic  iron  and  have  various 
features  similar  to  the  meteorites,  or  "shooting  stars,"  of  larger  size  which  fall 
on  the  earth's  surface. 

The  dark  cold  abysses  of  the  deeper  ocean  are  unfavorable  to  life,  yet  it  is 
there.  There  is  no  ground  vegetation,  and  the  animals  living  on  the  bottom 
must  exist  upon  the  sunken  bodies  of  the  plant  and  animal  life  living  in  the 
upper  layer  of  the  ocean  waters.  The  bottom  animals  are  of  rather  simple  types, 
such  as  certain  worms  and  star-fishes,  some  of  which  are  found  at  great  depths; 
they  exhibit  a  want  of  color  and  are  often  blind.  Some  have  in  themselves  the 
means  of  generating  light  by  phosphorescence. 

No  formations  have  yet  been  generally  found  among  the  stratified  beds, 
which  now  occupy  land  surfaces,  that  are  exactly  comparable  to  the  deposits 
now  found  on  the  floor  of  the  deep  ocean  basins,  and  the  conclusion  has  been 
drawn  from  this,  that  the  present  continental  areas  have  always  existed  as 
such,  and  have  never  been  sunk  far  enough  to  become  the  bottom  of  the  very 
deep  ocean  basins,  but  only  of  relatively  shallow  water  areas.  Some  limited 
occurrences  have  been  found  which  are  held  to  be  of  this  nature,  but  in 
amount  these  are  not  more  than  one  per  cent  of  the  continents. 

Islands.  —  These  may  be  divided  into  two  classes,  according  to 
the  positions  which  they  occupy,  continental  and  oceanic.  The 
former  are  related  to  the  continental  masses,  and,  in  general,  they 
rest  upon  the  continental  shelves,  and  are  near  the  mainland.  Long 
Island  on  the  Atlantic  coast  is  an  example.  They  may  be  formed  in 
two  ways ;  the  first  and  most  important  is  by  the  submergence  of  an 
irregular  coastal  land  surface,  whose  higher  portions  form  the 
islands.  Those  about  the  coasts  of  Maine  and  Norway,  see  Fig.  78, 
are  examples.  Subsequently  the  shapes  of  such  islands  may  be 
modified  by  wave  erosion.  Although  generally  small,  some  very 
large  islands,  like  Newfoundland  and  Great  Britain,  belong  to  this 
type.  Others,  like  the  East  Indies,  are  the  fragmented  remainders 
of  sunken  continents.  Still  a  third  way  in  which  continental  islands 
may  be  made  is  by  the  constructive  work  of  waves  and  currents, 
and  these  are  illustrated  by  the  low  sand  islands  thrown  up  as  bar- 
riers by  the  waves,  such  as  those  along  the  coast  of  the  southern 
states. 

The  true  oceanic  islands  are  those  rising  from  the  depths  of  the 
ocean  basins,  and  are  due  to  accumulations  on  their  bottoms.  Such 
accumulations  are  chiefly  volcanic  in  origin,  although  they  may  be 
greatly  added  to  by  organic  deposits  made  by  corals  and  other  ani- 
mals, as  described  under  organic  agencies.  Hawaii  is  an  excellent 
example  of  a  group  of  oceanic  islands  composed  of  lofty  piles  of 
volcanic  materials  rising  about  15,000  feet  from  the  bottom  of  the 


THE  OCEAN  AND  ITS  WORK  117 

deep  ocean  to  a  maximum  height  of  nearly  14,000  feet  above  its 
surface.  Samoa,  Bermuda  and  the  Azores  are  also  examples.  The 
Pacific  is  dotted  with  many  such  islands. 

In  contrast  with  these,  there  are  many  islands,  and  they  include  among 
them  some  of  the  largest,  like  New  Zealand,  whose  situation,  at  long 
distances  from  the  continents,  would  seem  to  place  them  in  the  same 
oceanic  class.  But  when  studied  we  find  that  the  nature  of  their  rocks 
and  the  geological  structures  which  they  display,  features  described  in 
the  second  part  of  this  work,  and  other  facts  lead  us  to  the  conclusion 
that  these  islands  are  to  be  regarded  as  parts  of  continental  masses, 
and  not  as  belonging  in  the  deep  ocean  basins.  Also  they  are  sur- 
rounded by  shallower  water  which  links  them  to  the  nearest  continental 
platforms.  Thus  Spitzbergen  is  linked  to  northern  Europe,  the  island  of 
South  Georgia  to  the  mainland  of  South  America,  and  a  group  consisting  of 
New  Guinea,  New  Caledonia,  Fiji,  New  Zealand,  Tasmania,  and  many 
smaller  islands,  appears  to  be  related  to  the  continental  mass  of  which  Aus- 
tralia is  the  largest  exposed  portion.  If  we  disregarded  origin  and  various 
geological  features,  and  went  merely  by  geographical  situation  on  the  map, 
many  of  these  might  be  classed  as  oceanic  islands,  but  their  true  relationship 
is  with  the  continental  platforms,  not  with  the  deep  ocean  basins. 


CHAPTER  V 
ICE  AS  A  GEOLOGICAL  AGENT 

By  far  the  greater  part  of  the  land  surface  of  the  globe  is  covered 
by  snow  and  ice  during  a  part  of  each  year  and  over  vast  tracts  of  it 
they  exist  perpetually.  Snow  and  ice  therefore  are  geological  fac- 
tors of  great  importance  whose  work  is  deserving  of  careful  atten- 
tion and  study.  The  only  difference  between  snow  and  ice  is  that 
the  former  consists  of  loose,  freely  grown  crystals  of  water,  whereas 
in  the  latter  the  crystals  form  a  compact  mass;  they  are  conse- 
quently to  be  considered  together,  especially  as  it  is  the  ice  that  is 
the  agent  of  importance. 

Ice  in  the  Soil.  —  The  work  of  frost  in  splitting  rocks,  and  thus 
helping  in  the  formation  of  soil,  has  been  already  explained  on  page 
21.  The  expansion  of  the  water  in  the  soil  in  freezing  also  causes 
movement,  and  a  variety  of  effects.  The  vertical  motion  due  to 
this  is  the  reason  why  posts  and  other  objects  buried  in  the  soil  are 
gradually  upheaved  and  overthrown.  On  slopes  it  produces  a  slow 
downward  creep  of  rocks  and  soils. 

The  reason  for  this  creep,  which  may  be  explained  by  the  aid  of  Fig.  90,  is 
as  follows:  When  the  soil  freezes,  a  bowlder 
lying  on  the  slope  would  be  lifted  by  the  expan- 
sion in  the  direction  ab  ;  on  thawing  taking  place 
it  will  sink  back  vertically  in  the  direction  be, 
thus  moving  down  the  slope  from  a  to  c.  Thus 
each  year  i^;  may  be  considered  to  take  a  step 


Fig.  90.  —  Showing  downward    down-hill,  and  the  sum  of  these  steps  produces 
creep  due  to  frost.  the  creep. 

Probably  it  is  due  in  part  to  this  expansion  and  movement  in 
forming  ice  that  heavy  falls  of  rock,  or  slumping  and  landslides,  take 
place  in  the  springtime  in  high  mountainous  regions,  especially  when 
aided  by  the  effect  of  heavy  rains.  Talus  slopes  thus  tend  by  creep 
and  sudden  sliding  to  move  downward,  and  where  they  are  extensive 
may  give  rise  to  streams  of  broken  rock  material,  especially  in  high 
mountains,  which  extend  down  into  valleys.  Such  rock  trains  have 
sometimes  been  called  rock-glaciers,  see  Fig.  91.  What  is  stated 

1J8 


ICE  AS  A  GEOLOGICAL  AGENT 


119 


later  in  regard  to  landslides  on  page  169  may  be  referred  to  in 
this  connection. 

River  Ice.  —  In  streams  which  freeze,  the  ice  becomes  a  consider- 
able factor  in  transporting  material,  often  of  considerable  weight 
and  size.  Along  the  shore  and  at  the  bottom  it  becomes  cemented 
into  stones  and  gravel  on  freezing,  and  along  steep  bluffs  consider- 
able masses  of  earth  and  rocks  may  fall  upon  it.  When  the  ice 
breaks  up  in  the  spring  more  or  less  of  this  may  be  carried  down- 


Fig.  91.  —  "Rock  Glacier"  in  Silver  Basin,  Colo.     Whitman  Cross,  U.  S.  Geol.  Surv. 

stream  attached  to  the  floating  ice  masses.  In  narrow  places  in  a 
river's  course  the  ice  cakes  may  become  jammed,  forming  ice  dams, 
by  which  the  muddy  water  of  the  stream  is  ponded  back,  overflow- 
ing the  adjacent  lowlands  and,  as  it  is  thus  brought  to  rest,  being 
made  to  deposit  its  load  of  sediment  on  the  alluvial  flats. 

Ice  in  Lakes.  —  The  chief  work  done  by  the  ice  in  lakes  is  ac- 
complished through  the  thrust  which  it  exerts  upon  the  shore  by  its 
expansion.  Ice,  like  other  substances,  is  expanded  and  contracted 
by  changes  of  temperature.  When  a  lake  freezes,  the  ice  cover  first 
formed  accurately  fits  its  surface;  if  the  temperature  falls  the 
ice  contracts  and  in  so  doing  cracks,  water  wells  upward  into  these 
cracks  and  is  frozen,  healing  the  cracks;  the  cover  of  ice  again  fits 
the  surface  of  the  lake  at  the  reduced  temperature.  If  the  latter 


120 


TEXT-BOOK   OF   GEOLOGY 


now  rises  the  ice  must  expand,  and  in  doing  so  exerts  an  enormous 
thrust  against  the  shore.  By  this  means  loose  material  is  pushed 
up  into  ridges,  and  bowlders  lying  in  shallow  water  within  reach  of 
the  ice  are  crowded  ashore,  forming  walls  of  stones,  or  ice  ramparts, 
about  the  border  of  the  lake,  as  illustrated  in  Fig.  92. 


Fig.  92.  —  Ice  rampart  on  Lake  Tenaya,  Calif.     G.  K.  Gilbert,  U.  S.  Geol.  Surv. 

The  Characters  of  Glaciers 

Perpetual  Snow-fields.  —  On  all  the  continents,  except  Australia, 
there  are  places  where  the  annual  fall  of  snow  is  not  entirely  dissi- 
pated each  year  by  evaporation  and  melting.  In  such  places  snow 
lies  upon  the  ground  all  the  year,  forming  perpetual  snow-fields.  In 
tropical  regions  this  occurs  only  on  the  tops  of  the  loftiest  moun- 
tains, in  temperate  regions  much  lower  down,  while  in  polar  lands 
such  snow-fields  approach  sea-level.  Thus  in  passing  from  the 
equator  to  the  poles,  and  depending  on  average  temperature,  there 
is  a  descending  line  or  surface  above  which  snow  lies  all  the  year, 
and  which  is  therefore  known  as  the  snow-line. 


At  the  equator  this  line  is  from  15,000-18,000  feet  high,  in  Mexico  14,000 
feet,  in  Colorado  12,000-13,000,  in  the  Yellowstone  Park  about  10,000-11,000, 
in  northern  Montana  about  9000,  in  southern  Alaska  about  5000,  in  southern 
Greenland  about  2000.  In  the  Alps  it  is  about  9000,  in  Norway  about  5000. 
The  snow-line  also  depends  very  much  upon  the  annual  precipitation;  where 
this  is  great  it  may  be  much  lower  than  in  places  with  a  corresponding  lati- 
tude where  the  snowfall  is  light.  Thus  at  the  western  end  of  the  Caucasus 
Mountains  on  the  Black  Sea  it  is  2000  feet  lower  than  on  the  eastern  end  of 
this  range  near  the  Caspian  Sea,  where  the  climate  is  much  drier.  In  Bo- 
livia under  the  equator  it  is  18,500  feet  on  the  dry  western  side  of  the  Andes, 


ICE  AS  A  GEOLOGICAL  AGENT 


121 


16,000  on  the  moister  east  side.  It  is  considerably  higher  in  Montana,  with  a 
rather  dry  climate,  than  in  Switzerland  in  the  same  latitude  with  a  moister 
one.  In  very  dry  regions  snow  may  disappear  more  rapidly  by  evaporation 
than  by  actual  melting. 

Neve ;  Change  into  Ice.  —  In  the  high  mountain  valleys,  slopes, 
and  amphitheatres  above  snow-line,  which  form  the  gathering 
ground,  or  catchment  basins,  for  the  perpetual  snow-fields,  through 
the  weight  of  the  accumulating  annual  layers  the  snow  becomes 
compacted,  and,  as  it  does  so,  it  changes  in  character.  From  the 
loose  feathery  condition  of  newly  fallen  snow,  it  assumes  a  granular 


Fig.  93.  —  The  gathering  ground  of  the  snows.    Nev6  fields  in  the  Mont  Blanc  region 

of  the  Alps. 

texture  like  rather  coarse  sand,  and  resembles  the  granular  snow 
which  we  are  accustomed  to  see  in  the  spring  as  the  remains  of  large 
drifts  from  the  winter.  This  is  largely  the  result  of  alternate  thaw- 
ing and  freezing  at  the  surface.  The  great  snow-fields  composed  of 
this  granular  snow  are  called  neve  slopes,  or  fields. 

Since  the  study  of  glaciers,  and  of  the  various  phenomena  associated  with 
them,  was  first  undertaken  in  the  Alps,  the  different  names  adopted  for  them 
are  largely  the  ones  used  by  the  French-  and  German-speaking  mountaineers. 
Neve  is  the  French  term,  firn  the  German  one. 


Beneath  the  surface  the  neve  rapidly  becomes  more  compact  and 
passes  into  porous  ice,  which  in  turn  becomes  denser.  This  ice  is 
more  or  less  distinctly  stratified,  or  in  banded  layers,  resulting  from 


122 


TEXT-BOOK    OF   GEOLOGY 


successive  falls,  or  annual  deposits,  having  somewhat  different  con- 
sistencies, or  being  separated,  or  outlined,  by  films  of  wind-blown 
dust  or  earth.  See  Fig.  94. 

Movement;  the  Glacier  Formed.  —  If  we  follow  the  neve  down 
its  slope,  toward  the  valley  below,  there  comes  a  point  in  the  mass  of 
accumulated  ice,  which  may  be  1000  feet,  or  even  much  greater  in 
thickness,  where  movement  begins,  and  the  ice  commences  to  flow 
slowly  down  the  valley  which  forms  the  outlet  of  the  catchment 
basin  above,  somewhat  after  the  manner  of  a  river.  It  flows  down 


Fig.  94.  —  Bergschrund,  and  beginning  of  the  glacier.     Mont  Blanc  region, 

Switzerland. 

the  valley  to  a  point  where,  eventually,  it  is  melted  and  changed  to 
a  river,  which  carries  off  the  surplus  drainage  of  the  basin.  The 
flowing  tongue  of  ice  projecting  from  the  upper  snow-fields  is  the 
glacier  proper.  Thus  in  regions  above  the  snow-line,  where  the  an- 
nual precipitation  is  mainly  in  the  form  of  snow,  the  drainage,  for  a 
certain  distance,  takes  place  in  part  in  the  form  of  ice,  giving  rise 
to  glaciers. 


The  point  at  the  surface  where  the  neve  ends  and  the  ice  of  the  glacier  is 
exposed,  is  at  the  snow-line  of  that  place.  This  is  usually  considered  the 
point  where  the  glacier  proper  begins,  but  movement  in  the  ice  underlying 
the  neve  fields  commences  far  above  this.  Very  often  there  is  a  wide  and 
deep  crack,  or  fissure,  or  a  number  of  them,  between  the  ice  of  the  snow-field 
and  the  adjacent  rock  surfaces  of  the  catchment  basin,  or  the  thinner  part  of 


ICE  AS  A  GEOLOGICAL  AGENT 


123 


the  neve  field  lying  on  them.  This  is  caused  by  the  initial  movement  of  the 
ice  mass,  and  is  known  as  the  bergschrund,  as  illustrated  in  Fig.  94. 

Not  every  snow-field  forms  a  definite  glacier;  frequently  it  is  not  sufficiently 
large  to  produce  ice  enough  to  cause  movement  to  be  generated  by  its  mass. 
It  is  simply  an  area  of  neve,  passing  into  ice  below.  Such  snow  patches 
are  common  in  all  high  mountains,  and  in  some  regions,  as  in  Colorado  for 
example,  where  the  combination  of  mountain  heights  and  amount  of  pre- 
cipitation is  not  capable  of  producing  snow-fields  large  enough  to  form  glaciers, 
they  alone  are  to  be  found. 

Another  stage  is  where  the  neve  fields  are  sufficiently  large  to  form  at  their 
lower  ends  ice  masses  which  show  by  various  features  that  movement  or 


Fig.  95.  —  A  glacieret.     The  Dana  glacier  in  1883.      Mt.  Dana,  Sierra  Nevada,  Cal. 

flowage  takes  place,  but  not  on  a  scale  which  enables  the  ice  to  project  any 
distance  below  snow-line,  or  to  produce  distinct  ice  tongues  flowing  down 
into  the  drainage  valleys.  Such  masses  are  variously  called  glacier ets,  hanging 
glaciers,  or  sometimes  cliff  glaciers,  when  nestled  in  the  face  of  a  cliff.  Every 
gradation  exists  between  simple  neve  patches,  glacierets,  and  glaciers  proper. 
The  so-called  glaciers  found  in  the  Rocky  Mountains  and  in  the  high  Sierras 
in  the  United  States  are  nearly  all  glacierets,  although  a  few  are  intermediate 
between  these  and  real  glaciers,  of  what,  as  we  shall  see  later,  are  called  the 
valley  type.  An  example  is  seen  in  Fig.  95. 

A  reconstructed,  or  "recemented'  glacier  is  formed  where  the  movement 
carries  a  glacier  over  a  cliff,  and  the  mass  of  fallen  fragments  below  is  molded 
by  weight  and  refreezing  into  a  solid  mass,  which  flows  onward  as  a  new 
glacier. 


124  TEXT-BOOK   OF   GEOLOGY 

Lower  Limit  of  Glaciers.  —  The  point  to  which  a  glacier  may 
descend  below  the  snow-line  before  being  melted  depends  on 
several  circumstances.  It  is  obviously  a  question  between  the  rate 
of  supply  and  that  of  melting,  and  might  be  likened  to  the  distance 
a  rod  of  ice  could  be  thrust  into  a  furnace  before  being  melted ;  this 
would  depend  on  the  size  of  the  rod,  the  rapidity  with  which  it  is 
pushed  forward,  and  the  heat  of  the  furnace.  Thus  in  tropical  and 
warm  regions  glaciers,  in  general,  project  but  a  short  distance  be- 
low the  snow-line;  as  we  go  farther  north,  although  the  snow-line 
descends,  we  also  find  glaciers  pushing  downward  a  relatively 
greater  and  greater  distance  from  it;  and,  eventually,  as  we  ap- 
proach sub-arctic  regions,  we  discover  them  entering  the  sea,  and 
ending  by  breaking  off  in  icebergs.  The  lower  limit  is  also  influenced 
by  climatic  conditions  for,  in  moist  regions,  there  is  a  greater  pre- 
cipitation and  supply,  hence  glaciers  are  larger,  more  rapid,  and  de- 
scend greater  distances  below  snow-line.  Locally  also,  in  a  given 
region,  a  large  glacier,  especially  if  confined  to  a  narrow  channel, 
descends  lower  than  a  small  one.  As  with  rivers,  so  in  these  streams 
of  ice,  if  they  can  go  far  enough,  as  in  Arctic  regions,  the  ultimate 
limit  is  the  sea. 

In  the  Alps  glaciers  project  as  far  as  5000  feet  below  the  snow-line,  and  in 
Norway  nearly  an  equal  distance.  In  southern  Alaska  they  come  down  to  sea- 
level  at  about  55°  N.,  and  also  in  southern  Greenland  at  about  60°,  whereas  in 
Norway,  which  extends  up  to  70°  N.,  they  fail  to  enter  the  sea  owing  to 
climatic  influences,  especially  of  the  Gulf  Stream,  see  page  94.  In  the  southern 
hemisphere,  in  New  Zealand,  which  has  many  fine  glaciers  in  the  Southern 
Alps  of  the  South  Island,  they  descend  in  latitude  45°  S.  into  sub-tropical 
forests  in  which  the  tree-ferns  spread  their  graceful  foliage,  and  in  southern 
Chile,  fed  from  the  Andes,  they  touch  sea-level  about  47°  S. 

Classes  or  Types  of  Glaciers.  —  According  to  the  size  and  fea- 
tures which  they  possess,  the  ice-fields  of  the  land  surface  of  the 
world  may  be  divided  into  three  great  classes,  or  types:  valley,  or 
Alpine,  glaciers;  piedmont  glaciers;  and  continental  glaciers,  or  ice- 
caps. Each  of  these  may  be  considered  separately  with  examples, 
since  each  is  of  importance  in  the  character  of  the  geological  work 
which  it  performs,  as  will  appear  presently. 

Valley  Glaciers.  —  These  are  of  the  class  which  has  already  been 
described  in  the  foregoing,  and  are  essentially  the  type  one  com- 
monly has  in  mind  when  glaciers  are  mentioned.  They  consist 
essentially  of  a  catchment  basin,  or  area,  for  the  gathering  of  snow 
above,  Fig.  96,  which  feeds  a  stream  of  ice  flowing  slowly  down  a 
valley,  and  which  is  the  glacier  proper,  Fig.  97,  until  through  melting 


TEXT-BOOK  OF  GEOLOGY 


125 


Fig.  96.  —  View  illustrating  the  lower  limit  of  glaciers  in  the  Alpine  type.  Glacier 
proper  begins  where  snow-line  ends.  Glacier  des  Bossons  descending  from  Mont 
Blanc,  Chamounix,  Switzerland. 


Fig.  97.  —  Typical  valley  glacier  with  branches;  lower  part  of  the  Mer  de  Glace 
in  1875.  Moraines  of  earth  are  seen  on  its  surface  as  dark  bands.  Chamounix, 
Switzerland. 


126  TEXT-BOOK   OF   GEOLOGY 

it  changes  to  a  river.  Such  glaciers  may  be  compared  to  rivers, 
and,  as  we  shall  see  presently,  they  have  many  points  in  common 
with  them,  and  some  marked  dissimilarities.  Like  rivers,  they  may 
have  tributaries,  that  is,  they  may  be  formed  of  a  number  of  ice 
streams  flowing  together  and  coalescing  in  a  final  trunk  glacier,  as 
illustrated  in  Fig.  97.  They  are  the  kind  of  glacier  found  in  the 
Alps,  where  glaciers  were  first  studied,  and  hence  are  often  called 
Alpine  glaciers;  they  are  the  ones  characteristic  of  all  high  moun- 
tain regions,  rising  above  the  snow-line. 

It  has  been  estimated  that  there  are  about  2000  glaciers  in  the  Alps; 
although  most  of  these  are  small  and  of  less  than  a  mile  in  length,  a  few  are 
from  three  to  five  miles,  and  one  is  ten  miles  long.  The  average  thickness  is 
probably  a  few  hundred  feet,  the  width  in  the  largest  a  mile.  In  Norway 
there  are  several  large  plateaus  of  ice  which  send  down  glaciers  in  several 
directions  into  the  valleys  below.  In  the  United  States  valley  glaciers  of 
some  size  are  found  only  on  the  lofty  volcanic  peaks  of  the  Cascade  Range 
in  northern  California,  Oregon,  and  Washington,  as  on  Mts.  Shasta,  Rainier 
(Tacoma),  Hood,  Baker,  Three  Sisters,  etc.  On  Shasta  they  attain  a  length 
of  two  miles,  on  Rainier  they  are  larger,  up  to  nearly  seven  miles.  Fine 
glaciers  are  found  in  the  mountains  of  British  Columbia,  and  northward  into 
Alaska,  where  they  are  of  great  size,  the  Seward  glacier  being  stated  as  50 
miles  long  and  three  miles  wide  in  its  narrowest  part. 

In  addition  to  the  various  regions  previously  mentioned,  magnificent  glaciers, 
up  to  30  miles  in  length,  are  found  in  the  Himalayas,  while  the  Caucasus, 
Nan  Shan,  and  other  high  ranges  of  Asia,  except  the  Altai,  furnish  fine 
examples.  On  the  high  lands  and  islands  of  both  the  Arctic  and  Antarctic 
oceans  they  abound,  and  even  in  Africa,  under  the  burning  equatorial  sun, 
small  ones  exist  on  Kilimandjaro  (20,000  feet),  and  on  Kenia  (19,500  feet),  and 
in  South  America  on  the  high  Andes  of  Ecuador. 

Piedmont  Glaciers.  —  If  valley  glaciers,  or  a  number  of  them, 
descend  far  enough  out  of  the  mountains  into  more  level  country 
below,  they  may  give  rise  to  a  large  area  of  nearly  stagnant  ice 
before  melting.  Such  an  ice-field  is  known  as  a  piedmont  (foot  of 
the  mountain)  glacier.  If  we  compare  valley  glaciers  to  rivers  a 
piedmont  glacier  might  be  likened  to  a  lake,  a  place  where  the  ex- 
cess of  ice  through  flowage  becomes  ponded,  before  melting  from  its 
surface  and  borders.  Such  glaciers  are  not  common,  and  are  con- 
fined to  regions  of  high  latitudes,  the  best  known  being  the  Malas- 
pina  glacier  in  Alaska. 

The  expanded  foot  of  the  Rhone  glacier  shown  in  Fig.  107  may  be  consid- 
ered as  the  beginning  of  a  piedmont  glacier,  and  illustrates  the  principle  of 
its  formation. 

The  Malaspina  glacier  is  situated  at  the  foot  of  Mt.  St.  Elias  (18,000  ft.)  and 


ICE  AS  A  GEOLOGICAL  AGENT 


127 


other  high  mountains,  which  feed  it  by  their  ice  streams.  It  covers  an  area 
of  1500  square  miles,  and  is  from  1500  to  1000  feet  thick.  As  may  be  seen 
from  the  map,  Fig.  98,  it  closely  borders  the  sea.  It  has  a  nearly  level, 
broadly  rolling  surface,  broken  by  innumerable  fissures.  Its  borders  are 
covered  with  earth  and  stones,  like  those  of  other  glaciers  and  described  later 
under  moraines  (p.  134),  and  these  deposits  in  places  are  deep  and  extensive 
enough  to  support  areas  of  dense  forest  growth,  although  resting  on  ice  1000 
feet  thick.  By  its  melting  the  glacier  gives  rise  to  several  rivers,  the  delta 
of  one  of  which  is  seen  in  Fig.  43. 

The  Muir  glacier  in  southern  Alaska,  at  the  head  of  the  inland  passage,  may 
be  taken  as  a  type  intermediate  between  the  piedmont  and  valley  glaciers.    It 


Fig.  98.  —  Map  of  the  Malaspiua  glacier,  after  I.  C.  Russell. 

fills  a  great  basin  of  about  350  square  miles  and  is  fed  by  ice  streams  coming 
from  the  high  mountains  which  surround  it.  The  lower  end  of  the  valley  basin 
touches  the  sea  and,  like  a  lake  with  accelerated  movement  of  the  water  at 
its  outlet,  the  ice  in  motion  discharges  into  the  head  of  Glacier  Bay  between 
mountain  walls,  forming  an  ice-cliff  two  miles  long,  from  which  icebergs  are 
continually  breaking  off,  with  an  uproar  like  heavy  thunder.  At  the  time  of 
the  writer's  visit  in  1887  the  ice-cliff  rose  to  a  height  of  about  250  feet  out  of 
the  sea;  the  depth  in  front,  as  ascertained  by  soundings  then  made,  was  over 
700  feet,  so  that  the  thickness  of  ice  was  not  less  than  1000.  A  view  of  this 
ice  front  is  seen  in  Fig.  99.  The  Muir  is  one  of  the  largest  of  what  have 
been  called  " tide-water"  glaciers,  those  which  reach  the  sea. 

Continental  Glaciers ;  Ice-caps.  —  These,  as  their  name  implies, 
are  ice  sheets  of  vast  extent.    If  the  two  former  classes  of  glaciers 


128  TEXT-BOOK    OF   GEOLOGY 

may  be  compared  to  rivers  and  lakes,  these  may  be  termed  seas  of 
ice.  Their  chief  feature  is  the  almost  endless  monotony  of  broadly 
rolling,  nearly  level  surfaces  of  wind-swept  ice  which  they  present, 
a  monotony  varied  only  by  the  continually  successive  storms  of 
snow  which  maintain  them.  Near  their  borders  they  are  often 
varied  by  occasional  mountains  of  rock,  rising  like  islands  through 
the  ice  and  known  as  nunataks.  At  their  borders  they  thin  down 
into  prolonged  lobes,  or  give  rise  to  definite  streams  of  ice  which 
discharge  into  the  ocean,  forming  icebergs.  Only  two  examples  of 


Fig.  99.  —  Sea-cliff  of  the  Muir  glacier  in  1887. 

them  exist  today,  in  the  ice-caps  which  cover  Greenland  and  the 
continent  of  Antarctica,  upon  the  latter  of  which  the  South  Pole  is 
situated. 

The  ice-cap,,  or  inland  ice,  of  Greenland  covers  an  area  of  probably  715,000 
square  miles.  It  has  been  traversed  by  Nansen  and  Peary  and,  according  to 
these  explorers,  the  great  shield  of  ice  rises  to  a  height  of  8500  feet.  Its 
thickness  is  not  known,  but  may  be  several  thousand  feet.  This  ice  is  in 
motion,  as  shown  at  its  edges,  where  it  becomes  more  rapid  as  it  descends  in 
valleys  to  the  sea,  but  the  movement  in  the  interior  must  be  almost  indefi- 
nitely slow. 

The  size  of  the  Antarctic  ice-cap  is  not  known,  but  it  is  thought  to  be  nearly 
5,000,000  square  miles  in  area.  It  has  been  partly  explored  by  Amundsen,  Scott 
and  Shackleton  in  their  journeys  to  reach  the  South  Pole.  It  attains  a  height 
of  9000  feet.  Like  the  ice  sheet  of  Greenland  it  thins  towards  the  sea,  and 
descending  through  valleys  in  the  mountain  rim  gives  rise  to  huge  moving 
glaciers.  According  to  Scott  it  pushes  off  the  land,  and,  advancing  on  the 
sea,  covers  the  latter  over  vast  stretches  with  a  floating  field  of  ice,  known  as 
the  'Great  Ice  Barrier/  from  whose  front,  by  their  breaking  off,  the  great 
tabular  bergs  of  the  Antarctic  Ocean  are  formed.  Other  great  ice-caps  sim- 
ilar to  these  have  existed  in  the  past,  but  have  melted  and  disappeared.  This 
is  clearly  shown  by  the  geological  work  they  performed,  as  we  shall  see  later. 


ICE  AS  A  GEOLOGICAL  AGENT  129 

Various  Features  of  Glaciers 

Movement.  —  The  movement  of  glaciers,  as  compared  with 
rivers,  is  very  slow.  Their  motion  was  not  generally  known  until 
the  early  part  of  the  last  century  when  it  was  observed  that  a  hut 
built  upon  one  of  the  Alpine  glaciers  changed  its  position,  and  the 
amount  of  change  and  the  rate  were  measured.  Since  then  this 
subject  has  been  much  studied,  and  a  great  deal  learned  concerning 
the  nature  of  the  motion  of  ice  in  glacial  flowage.  In  the  Alps  the 
glaciers  have,  in  general,  been  found  to  move  from  one  to  three  feet 
a  day,  or  from  about  300  to  1000  feet  a  year,  and  this  may  be  taken 
as  about  the  rate  of  ordinary  valley  glaciers.  The  Muir  glacier  at 
its  outlet  has  been  found  to  move  seven  or  more  feet  a  day,  or  at  a 
rate  of  2500  feet  per  annum,  while  some  Greenland  glaciers  have 
been  measured  up  to  60,  or  even  more,  feet  per  day,  but  in  these 
cases  the  ice  of  great  interior  areas  is  pushed  with  accelerated  mo- 
tion through  valley  openings  into  the  sea.  It  has  been  found 
that  the  rate  is  influenced  by  several  factors ;  thus  it  increases  with 
a  steeper  slope  and  smoother  bed ;  it  is  faster  when  the  ice  is  thicker, 
and  it  is  more  rapid  in  summer,  when  it  is  warmer  and  the  ice  is 
melting,  than  in  winter;  the  gradient,  the  thickness  of  ice,  and  the 
temperature  are  thus  the  chief  things  which  affect  the  rate  of 
motion. 

Differential  Motion.  —  One  of  the  most  important  facts  which 
has  been  discovered  in  regard  to  the  motion  of  a  glacier  is  that  it  is 
not  the  same  in  all  parts  of  its  mass.  The  glacier  does  not  move  by 
sliding  down  its  bed  as  a  whole,  bodily,  like  a  cake  of  ice  off  the 
roof  of  a  house.  This  is  evident  since  they  often  move  on  slopes 
elevated  only  a  few  degrees  from  a  level  surface,  and  in  the  case  of 
the  great  ice-caps  the  flowage  probably  takes  place  outward  from 
the  center  owing  to  the  thickness  of  the  accumulated  ice  mass,  as 
much  as,  if  not  more  than,  because  of  the  slope  of  the  land.  It  has 
been  found  by  driving  rows  of  stakes  aligned  across  the  glacier  with 
some  point  on  either  side,  and  observing  their  line,  that,  after  a 
lapse  of  time,  the  line  is  curved  downstream  and  therefore  the  cen- 
ter moves  faster  than  the  sides.  In  a  similar  way,  on  an  exposed  side 
of  the  glacier,  a  vertical  line  of  pegs  proved  that  the  top  moves  faster 
than  the  bottom.  It  has  also  been  observed  that  there  is  a  line  of 
swiftest  motion,  more  sinuous  than  that  of  the  valley  in  which  the 
glacier  lies,  or  that  curves  in  its  banks  reflect  the  current  back  and 
forth,  as  in  a  river.  (See  page  66.)  The  motion  of  the  ice  is  there- 
fore differential,  that  is,  some  parts  of  the  ice  are  moving  over  and 


130  TEXT-BOOK   OF   GEOLOGY 

past,  and  therefore  faster  than,  other  parts.  In  these  respects  a 
glacier  is  like  a  river,  and  the  ice  indeed  appears  to  move  as  if  it 
were  a  thickly  viscous  fluid,  like  pitch  or  asphalt,  which,  though 
brittle  enough  to  be  broken  by  a  sudden  blow,  yields  under  the 
pressure  of  its  own  weight,  and  undergoes  slow  flowage. 

Origin  of  Glacier  Motion.  —  The  behavior  of  ice  in  exhibiting 
flowage  in  glaciers  has  been  the  subject  of  much  investigation  and 
discussion.  It  is  clear  that  the  force  which  causes-  it  is  the  weight 
of  the  ice,  due  to  gravity,  and  this  vertical  downward  force  may  be 
resolved  into  components,  one  of  which  tends  to  thrust  the  ice  in  the 
direction  of  the  slope  on  which  it  lies.  It  is  also  clear  from  the 
differential  motion  that  the  ice  is  not  sufficiently  rigid  to  resist  this 
thrust,  but  is  plastic  in  response  to  it,  moving  over  itself,  and 
dragging  the  whole  mass  downward.  It  is  also  known  that  glacial 
ice  has  a  granular  structure,  consisting  of  interlocked  crystal  grains, 
which  may  be  as  large  as  peas  or  larger,  and  is  thus  in  its  texture 
like  granite  or  some  other  similar  rock. 


Fig.  100.  —  Diagram  to  show  force  causing  glacial  motion. 

The  granular  condition  begins  in  the  neve  fields  and  persists,  the  granules 
becoming  compacted  together  into  solid  ice,  but  growing  larger.  In 
the  compact  massive  ice  of  the  glacier  the  fact  that  it  is  composed  of  irregu- 
larly shaped  granules  closely  fitted  together,  like  a  rock,  is  not  perceptible  to 
the  eye,  but  the  structure  may  be  clearly  seen  when  a  thin  sheet  of  the  ice  is 
placed  in  an  apparatus  suitable  for  observing  it  in  polarized  light.  Each 
grain  then,  in  general,  appears  of  a  different  color  from  its  neighbors,  and 
the  structure  is  plainly  blocked  out.  At  the  lower  end,  also,  through  differ- 
ential melting,  the  grains  may  fall  apart  into  gravel  or  sand-like  heaps. 

The  influence  of  gravity  on  glacial  motion  may  be  considered  as  follows: 
let  ABCD,  in  Fig.  100,  be  a  portion  of  a  glacier  resting  on  the  slope  CD. 
Let  the  vertical  line  gr  in  its  direction  and  length  represent  the  force  of 
gravity  acting  on  it.  Draw  gt  and  sr  perpendicular  to  CD  and  AB.  In  the 
parallelogram  of  forces  srgt  the  component  gs  represents  that  portion  of  the 
force  of  gravity  which  thrusts  the  ice  in  the  direction  BA.  The  resistance  to 
this  force  is  composed  of  the  friction  on  the  bed  CD  and  the  backward  thrust 
of  any  surface  AC.  If  a  mass  ABE  were  entirely  composed  of  ice  resting  on 
a  level  surface  AE,  the  tendency  of  a  particular  layer  ABCD  to  move  in  the 
direction  BA  in  response  to  the  thrust  gs  would  be  opposed  by  the  resistance 
to  shearing  along  the  line  CD.  The  differential  motion  which  occurs  shows 
that  this  resistance  is  not  sufficient  to  withstand  the  thrust.  Since  the  ice  is 


ICE  AS  A  GEOLOGICAL  AGENT 


131 


composed  of  grains,  these  grains  tend  to  revolve  as  shown  in  the  diagram,  e 
and  e;  this  tendency  is  resisted  by  the  irregular  interlocked  form  of  the 
grains  and  the  rigidity  of  the  crystal  ice  composing  them. 

Apparent  Viscosity  of  Ice.  —  It  was  formerly  thought  that  ice 
was  a  viscous  substance,  and  that  when  in  mass  it  exhibited  the 
property  of  slowly  yielding  to  its  weight  and  of  flowing,  that  such 
bodies  possess,  as  seen  in  pitch  and  asphalt.  This  apparent  be- 
havior of  the  ice  in  a  glacier  has  been  mentioned  previously.  We 
now  know  that,  however  much  ice  may  apparently  show  this  prop- 
erty, it  is  not,  and  cannot  be,  truly  viscous,  and  that  the  apparent 
viscosity  must  be  explained  in  some  other  way. 


A« 

cl, 

E«: 


Fig.  101.  —  Diagram  illustrating  gliding  planes  in  an  ice  crystal.      The  lower  figures 
illustrate  the  molecular  structure  in  a  vertical  plane. 

The  difficulty  in  entertaining  this  view  of  viscosity  is  that  ice  is  crystalline 
in  structure,  and  in  crystals  the  physical  molecules  are  arranged  in  definite 
geometrical  positions  in  space  with  respect  to  one  another,  and  to  break  up 
this  arrangement  would  destroy  the  physical  identity  of  the  substance,  as 
when  ice  changes  to  water.  In  a  viscous  substance  the  molecules  on  the  con- 
trary have  no  definite  arrangement,  and  can  occupy  any  position  with  respect 
to  one  another  without  destroying  its  physical  state.  This  is  a  fundamental 
property  of  liquids,  and  viscous  substances  may  be  regarded  as  very  stiff 
liquids.  Cold  molasses  is  a  good  example  of  a  viscous  fluid.  Hence  ice,  being 
crystalline,  cannot  exhibit  the  property  of  true  viscosity. 

Explanation  of  Glacial  Movement.  —  A  variety  of  different 
theories  have  been  proposed  to  explain  the  apparent  viscosity  of 
ice  and  glacial  motion,  which  it  would  be  beyond  the  scope  of  this 
work  to  discuss.  We  must,  therefore,  content  ourselves  with  what, 
in  the  light  of  our  present  knowledge,  seems  the  most  probable  ex- 
planation. This  depends  on  two  properties. of  ice;  first,  that  ice  be- 
low the  freezing  point  if  subjected  to  sufficient  pressure  will  melt 
and,  if  the  pressure  is  removed,  will  refreeze;  and  second,  that  in  a 


132 


TEXT-BOOK   OF   GEOLOGY 


certain  direction  one  part  of  an  ice  crystal  can  be  pushed  over  an- 
other part  of  the  crystal,  without  destroying  its  crystalline  nature. 

With  regard  to  the  first,  the  freezing  point  of  water  is  lowered  by  pressure. 
This  depends  on  the  fact  that  water  expands  in  changing  to  ice.  Thus 
under  atmospheric  pressure  water  freezes  at  32°  F.;  if  we  put  sufficient  pres- 
sure upon  it  the  temperature  may  be  lowered  to  30°  and,  since  it  cannot 
expand  into  ice,  it  will  still  remain  liquid;  should  the  pressure  be  removed  it 
will  at  once  freeze.  Conversely  if  we  place  ice  under  sufficient  pressure  at  a 
given  temperature,  say  31°  F.,  it  will  contract  by  turning  into  water;  if  the 
pressure  be  relieved  it  will  immediately  revert  to  ice.  At  the  bottom  of  5000 
feet  of  ice  the  melting  temperature  is  about  302°  F.,  or  —  1°  C. 


Fig.  102.  —  Erratic  block  mounted  on  ice  pedestal;    caused  by  differential  melting. 

Switzerland. 

With  regard  to  the  second  property  it  has  been  found  by  experiment  that 
ice  crystals  have  the  quality,  when  subjected  to  a  sufficient  shearing  force  in 
a  certain  direction,  as  along  the  plane  CD  in  the  crystal  ABEF,  Fig.  101,  of 
the  molecules  being  able  to  slip,  or  glide,  along  this'  plane  and  change  po- 
sition without  destroying  the  physical  condition,  or  cleaving  or  breaking 
the  ice.  Thus  the  part  GB'HE'  in  Fig.  101,  illustrating  the  molecular  struc- 
ture, is  as  firm  and  solid  an  ice  crystal  as  before.  This  slipping  can  take  place 
between  any  layer  of  molecules  in  a  plane  parallel  to  the  base  of  the  hexagonal 
prism,  but  not  in  any  other  direction.  A'F'  represents  a  crystal  thus 
partly  deformed.  A  crystal  which  has  this  property  is  said  to  have  gliding 
planes;  it  is  possessed  by  various  substances,  such  as  the  (jommon  mineral 
calcite,  CaCO3. 

In  applying  these  properties  to  explain  glacial  movement  we  can 
see  that  the  resistance  to  revolution,  which,  by  the  thrust  of  gravity, 
would  generate  forward  motion  of  the  interlocked  crystal  grains  of 


ICE  AS  A  GEOLOGICAL  AGENT  133 

ice,  as  a  mass  of  shot  would  flow  down  an  inclined  trough,  may  be 
overcome  by  the  pressure.  The  minute  points  of  resistance  on  each 
grain  may  be  momentarily  liquefied  by  the  pressure,  aided  by 
the  heat  of  friction,  the  grain  revolved  more  or  less  and  changed  in 
position,  and,  the  stress  being  relieved,  the  water  would  immediately 
resolidify.  And,  whenever  a  grain  happens  to  be  in  the  right  posi- 
tion, so  that  the  thrust  is  in  the  direction  of  the  gliding  planes, 
motion  will  take  place  along  them  also.  Since  the  latter  is  the  easier 
way  of  relieving  stress,  whenever  a  grain  comes  into  this  position  it  is 
likely  to  stay  there;  and  hence  in  the  lower  part  of  the  glacier,  and 
towards  its  end,  the  crystals  are  largely  in  parallel  position  and  the 
movement  is  chiefly  along  gliding  planes.  Thus  by  a  combination 
of  melting  and  refreezing  under  pressure,  and  by  slippage  along 
gliding  planes,  the  granules  are  able  to  change  position,  and  the 
ice  to  thus  undergo  a  slow  flowage  which  simulates  a  viscous  motion. 

Surface  of  a  Glacier.  —  The  surface  of  a  glacier  is  not  ordinarily 
smooth  and  unbroken  like  that  of  a  frozen  river;  on  the  contrary  it 
usually  has  a  variety  of  features  which  make  travel  over  it  ex- 
tremely difficult.  Aside  from  minor  irregularities  produced  in  a 
variety  of  ways  by  unequal  melting,  one  phase  of  which  is  illustrated 
in  Fig.  102,  the  ice  is  traversed  by  wide  and  deep  cracks  called 
crevasses,  and  covered  in  places  by  accumulated  heaps  of  earth  and 
stones  termed  moraines.  Each  of  these  deserves  consideration. 

Crevasses.  —  These  fissures  may  have  any  width,  up  to  20  feet, 
or  even  more,  and  of  great  depth,  100  feet  or  greater.  The  most 
prominent  ones  are  transverse  to  the  course  of  the  glacier  and  are 
caused  by  the  passage  of  the  ice  stream  over  a  salient  angle  in  its 
bed,  with  change  from  a  lesser  to  a  greater  gradient,  as  illustrated 
in  Fig.  103.  When  the  ice  passes  over  such  a  prominence,  tension  is 
produced,  greater  in  the  upper  than  in  the  lower  layers  of  ice;  the 
latter  yields  to  the  tension  and  cracks;  the  fissures  yawn  widely 
at  the  top  and  gradually  close  be- 
low. They  also  curve  downstream, 
since  the  motion  is  swifter  in  the 
center,  and  this  tends  to  bend  them. 
Even  a  change  of  angle  in  the  bed 
of  only  two  or  three  degrees  pro-  Fie-  103-  —  Diagram  to  show  the 

i  .    .,  .  f  formation  of  crevasses. 

duces   crevasses,   a   striking  proof 

that  ice  is  not  a  truly  viscous  substance.  Where  a  very  steep  gra- 
dient is  encountered  the  ice  is  much  broken  and  an  ice-fall  is 
produced,  as  illustrated  in  Fig.  107  of  the  Rhone  glacier.  The 
pointed  jagged  masses  of  ice  made  by  crevassing  are  called  seracs. 


134  TEXT-BOOK   OF   GEOLOGY 

Crevasses  also  occur  on  the  margin  where  the  ice  drags  against  the 
inequalities  of  the  side  walls  of  the  valley.  These  marginal  cre- 
vasses point  inward  and  upward  at  about  45°  with  respect  to  the 
course  of  the  glacier.  Longitudinal  ones  also  occur  in  the  terminal 
lobe  of  the  glacier  where  the  ice,  relieved  from  transverse  pressure, 
tends  through  lateral  spreading  to  fall  apart,  as  illustrated  in  Fig. 
107.  The  first  crevasse  which  forms  in  the  neve  field,  and  which 
shows  the  initiation  of  movement,  is  called  the  bergschrund,  as  pre- 
viously mentioned,  page  123.  The  transverse  crevasses  once  formed 
do  not  remain  indefinitely,  for  on  moving  over  a  salient  angle  the 
ice  generally  passes  into  a  re-entrant  one,  as  illustrated  in  Fig.  103, 
the  tension  is  replaced  by  compression,  which  more  or  less  closes  the 
fissures,  the  ice  blocks  refreeze,  and  the  crevasses  may  be  obliterated, 
though  generally  not  completely,  owing  to  enlargement  by  melting. 
Thus  the  glacier  in  its  course  is  subjected  in  places  to  crevassing, 
which  may  disappear  elsewhere,  somewhat  as  a  river  at  its  rapids 
may  display  foamy  water,  not  elsewhere  seen. 

Moraines.  —  In  an  ordinary  valley  the  debris  of  earth  and 
stones,  which  is  produced  as  a  result  of  the  weathering  and  erosion 
of  its  sides  and  walls,  would  form  slopes  and  talus  slides,  which 
passing  downward  by  creeping  would  be  ground  up  by  the  stream 
and  borne  away.  In  a  valley  more  or  less  filled  by  a  glacier  this 
material  descends  instead  upon  the  ice  and  is  taken  along  by  it  as 
bands  of  heaped-up  material.  Sometimes,  however,  it  falls  into  the 
bergschrund  and,  becoming  frozen  into  the  ice,  is  carried  away. 
Moreover,  the  ice  at  the  bottom  of  the  neve  fields,  being  frozen  into 
cracks  and  cavities,  and  around  projections  in  its  stony  bed,  when 
motion  begins,  "plucks"  or  quarries  masses  of  rock  and  takes  them 
forward  with  it.  All  the  material  thus  obtained  and  transported  by 
the  glacier  serves  to  form  the  moraines.  If  the  material  is  carried 
on  top  of  the  ice  it  is  spoken  of  as  super  glacial;  if  frozen  fast  within 
the  ice,  as  englacial;  if  transported  fixed  in  the  bottom  of  the  glacier, 
as  subglacial.  In  the  upper  neve  fields,  since  these  are  covered  by 
successive  snowfalls,  there  is  little  superglacial  material;  in  the 
lower  part  of  the  glacier  proper,  as  melting  and  waste  become  more 
and  more  pronounced,  a  greater  and  greater  quantity  of  englacial 
material  appears  at  the  surface  and  becomes  superglacial. 

Moraines  are  generally  divided  into  four  classes,  according  to  the 
position  they  occupy  with  respect  to  the  glacier,  as  lateral,  medial, 
ground,  and  terminal.  Lateral  moraines  are  formed  along  the  sides 
as  explained  above.  They  become  larger  and  more  evident 
toward  the  terminus,  forming  ridges  of  earth  and  stones,  25, 


ICE  AS  A  GEOLOGICAL  AGENT 


135 


50,  or  even  100  feet  high.  The 
medial  moraines  are  made  of  the  lateral 
ones  when  tributaries  come  in,  as  illus- 
trated in  Fig.  104,  and  they  may  be 
seen  in  the  view,  Fig.  97.  There  may 
be  as  many  as  eight  or  ten  of  them.  At 
the  lower  end,  through  melting,  as  men- 
tioned above,  englacial  material  appears 
at  the  surface  and  gives  rise  to  new 
medial  moraines.  See  Fig.  104.  The 
ground  moraine  consists  of  the  debris 
carried  along  at  the  bottom  of  the  glacier. 
All  of  the  material  transported  by  the 
ice  is  eventually  dumped  at  its  end  in  a 
confused  mass  of  earth  and  stones  which 
forms  the  terminal  moraine.  This  may 
coalesce  with  the  lateral  moraines  as  in- 
dicated in  Fig.  104.  Finally  it  must  be 
remembered  that  through  the  longer  part 
of  its  course  the  greater  part  of  the  ma- 
terial is  in  and  under  the  ice. 

Veins  and  Layers.  —  The  ice  of  gla- 
ciers frequently  has  a  veined,  or  mar- 
bled, appearance  due  to  bands  of  ice 
varying  in  color  from  blue  to  white.  The 
white  ice  is  full  of  minute  air  bubbles 
which  produce  the  color ;  the  blue  is  free 
from  them.  The  veining  is  formed  at 
places  of  greatest  compression,  as  where 
re-entrant  angle  in 


Fig.  104.  —  Plan  of  a  valley  gla- 
cier, showing  tributaries  and 
terminal  lobe;  II,  lateral  mo- 
raines running  into  tt,  the  ter- 
minal moraine;  mm,  medial 
moraines.  As  melting  pro- 
gresses, more  and  more  ma- 
terial appears;  s,  exit  of  sub- 
glacial  stream;  vv,  valley  train 
of  water-laid  debris. 


the  ice  moves  into  a 

its  bed,  and  the  pressure  squeezes  the  air 
out  of  the  white  vesicular  ice,  turning  it  into  blue,  which  the  on- 
ward motion  streaks  out. 

In  the  lower  part,  where  the  thrust  from  gravity  is  most  pro- 
nounced, the  ice  often  shears  and  is  pushed  over  itself,  forming  a 
series  of  distinct  layers,  or  bands.  This  banded  appearance  is  often 
greatly  enhanced  by  layers  filled  with  dirt  and  gravel,  which  con- 
trast with  others  of  clearer  ice.  This  often  gives  the  glacier  a  re- 
semblance to  beds  of  stratified  rock,  as  illustrated  in  Fig.  105. 

Drainage ;  Subglacial  Stream.  — Within  a  glacier,  in  the  region 
above  snow-line  where  it  is  growing,  the  temperature  must  be  below 
the  melting  point;  in  the  lower  part,  where  it  is  wasting,  at  the 
melting  point.  Hence  melting  is  generally  going  on  all  the  time ;  in 


136  TEXT-BOOK   OF   GEOLOGY 

summer  it  is  melting  rapidly  at  the  surface,  and  is  traversed  by 
streams  of  water,  which  fall  into  crevasses,  or  form  pools  on  its 
surface.  All  of  this  water  eventually  descends  to  form  a  stream 
under  the  glacier,  which  issues  at  its  lower  end,  sometimes  from  an 
ice-cave,  sometimes  from  along  one  side,  and  thus  carries  off  the 
general  drainage  of  the  valley,  Fig.  106.  Such  streams  are  very 
turbid,  being  heavily  charged  with  sediment,  and  as  this  sediment 
consists  of  fine  particles  of  fresh,  unweathered  rock,  ground  up  by 
the  glacier  on  its  bed,  they  are  chiefly  white  in  color  and  give  the 
stream  a  peculiar  milky  appearance,  which  it  may  retain  for  long 
distances.  This  milky  look  is  so  characteristic  as  to  lead  to  the 
suspicion,  when  seen  in  a  river,  that  it  is  fed  by  melting  glaciers 
higher  up  in  its  course. 


Fig.  105.  —  View  illustrating  the  veined  structure  of  a  glacier,  simulating  folded  rock 
strata.  The  situation  is  at  the  end  of  the  glacier,  and  the  subglacial  stream  may 
be  seen  appearing,  and  also  morainal  material.  Greenland.  W.  H.  Brewer. 


Advance  and  Recession  of  Glaciers.  —  We  know  in  a  variety 
of  ways  that  great  changes  of  climate  have  occurred  in  the  past,  as 
will  be  fully  discussed  in  the  historical  part  of  this  book.  These 
changes  have  been  not  only  general,  but  also  local,  as  affecting  some 
particular  region,  and  have  already  been  mentioned  in  one  aspect 
under  the  history  of  salt  lakes.  Such  changes  have  a  profound 
effect  upon  the  existence  of  glaciers.  Thus  we  know  from  evidence, 
to  be  shown  later,  that  in  recent  geological  times  North  America 
and  Europe  were  covered  with  great  continental  glaciers,  or  ice- 
caps, similar  to  that  of  Greenland  to-day,  as  far  south  as  Ohio  and 
middle  Germany.  At  the  same  time  the  valley  glaciers  of  the  Rocky 


ICE  AS  A  GEOLOGICAL  AGENT 


137 


Mountains  and  of  the  Alps  had  a  vast  extension  over  their  present 
size.  With  change  of  climate  these  ice-caps  have  disappeared,  and 
valley  glaciers  have  in  some  places  greatly  shrunken  and  in  others 
melted  away. 

But  aside  from  these  great  changes,  occupying  periods  of  geologic 
time,  glaciers  appear  to  grow  and  advance,  or  to  diminish  and  re- 
treat, in  response  to  varying  climatic  cycles  of  years  of  greater  pre- 
cipitation and  coldness,  compared  with  ones  of  greater  sunshine  and 


Fig.  106.  —  Subglacial  stream  and  ice-cave.  Morainal  material  is  seen  above  which 
falls  as  the  ice  melts  and  helps  to  build  the  terminal  moraine.  Transported  blocks 
fill  the  bed  of  the  turbulent  stream  which  carries  the  finer  earth  and  ground-up 
rock  away.  Chamounix,  Switzerland. 


warmth.  Thus  the  glaciers  of  the  Alps,  during  the  past  century, 
have  been  observed  in  a  number  of  cases  to  have  experienced  such 
oscillations,  but  up  to  the  present  an  insufficient  amount  of  accurate 
data,  relating  to  this  phenomenon,  has  been  gathered  to  enable  us 
to  state  definitely  the  periods,  and  the  laws  governing  them,  and 
whether  on  the  whole,  irrespective  of  these  periodic  oscillations, 
glaciers  the  world  over  are  diminishing  or  not.  At  the  present  time 
in  the  Alps  and  in  North  America  they  are  in  general  retreating,  as 
illustrated  in  Fig.  107  of  the  Rhone  glacier.  In  20  years  since  1887 
the  Illecillewat  glacier,  one  of  the  valley  type  in  British  Columbia, 
was  observed  to  have  retreated  at  least  500  feet.  Some  individual 


138 


TEXT-BOOK   OF   GEOLOGY 


glaciers  in  the  Alps  and  in  Alaska,  on  the  other  hand,  have  been 
shown  to  be  advancing.  In  1899  central  Alaska  was  visited  by  a 
heavy  earthquake  which  shattered  the  ice  of  many  glaciers  in  that 
region.  In  some  of  the  land  glaciers  this  caused  a  rapid  advance 

of  the  ice,  but  in  tide-water  gla- 
i  ciers,  like  the  Muir,  see  page  127, 
so  much  ice  was  broken  from 
the  front  and  floated  out  to  sea, 
that  notable  retreats  of  these 
fronts  took  place;  of  a  number  of 
miles,  in  two  or  three  years,  in 
the  case  of  the  Muir. 

Geological  Work  of  Glaciers 

The  work  of  glaciers,  like  that 
of  rivers,  consists  in  erosion, 
transportation  and  deposition. 
While  in  these  features,  speaking 
broadly,  they  are  like  rivers,  the 
manner  in  which  the  work  is 
done  and  the  results  achieved 
are  very  different,  as  will  appear 
from  the  following  discussion  of 
them.  Our  knowledge  in  this 
case  is  obtained  by  observation 
of  living  glaciers,  of  areas  which 
they  have  uncovered  and  aban- 
doned in  recent  retreats,  and  by 

Fig.  107.  —  Two  views  of  the  Rhone  gla-    application     of     the     f acts     thus 
cier,  Switzerland;  the  upper  one  taken    ,  -,   .  .  .          ,  .    ,     ,, 

in  1870,  the  lower  in  1905,  which  iiius-  Darned  in  regions  in  which  they 

trate  the  retreat  of  the  ice  in  35  years,  no  longer  exist,  but  which  these 
The  older  one  shows  the  terminal  lobe  evidences  show  they  once  OCCU- 
and  longitudinal  crevasses.  .  , 

pied. 

Glacial  Erosion.  —  In  its  highest  part,  under  and  at  the  edges 
of  the  neve  slopes,  a  glacier  erodes  chiefly  by  "plucking"  and  gathers 
its  load  by  this  process,  and  by  collecting  the  debris  coming  to  it 
through  the  weathering  of  the  slopes  above  the  snow,  as  described 
under  moraines.  The  work  of  gathering  is  especially  active  in  the 
bergschrund,  where  this  crevasse  comes  between  rock  and  snow;  in 
summer  time  thawing  takes  place  during  the  day,  the  rocks  are  wet, 
and  freezing  is  apt  to  occur  at  night,  springing  out  blocks  which  fall 


ICE  AS  A  GEOLOGICAL  AGENT 


139 


down  to  be  enveloped  by  the  ice  and  carried  away.  Thus,  over  the 
area  where  the  neve  fields  rest,  they  are  constantly  quarrying  inward 
and  downward,  and  in  the  long  course  of  time  this  gives  rise  to 
partly  bowl-shaped  valleys,  or  ba- 
sins, called  amphitheaters,  or 
cirques.  These  cirques  are  often 
cut  somewhat  deeper  at  the  center 
than  at  the  place  of  discharge  and 
thus,  when  the  valley  is  no  longer 
filled  with  ice  and  snow,  these  de- 


Fig.  108.  —  Section  through  amphi- 
theater and  glacial  lake. 


pressions  are  occupied  by  one  or  more  small  ponds  or  lakes,  as 
shown  in  Fig.  108.  Such  cirques  are  common  features  in  the  moun- 
tains of  northern  regions  and  exhibit  the  collecting  basins  of  former 
glaciers.  See  Figs.  109  and  116. 

As  soon  as  movement  begins,  the  erosion  is  somewhat  different. 
Plucking  continues,  but  in  addition  the  earth  and  stones  frozen  fast 


Fig.  109.  —  View  of  glacial  cirque  or  amphitheater.     Sultan  Mountain,  Colo.     F.  L. 
Ransome,  U.  S.  Geol.  Surv. 

into  the  bottom  of  the  ice  form  a  huge  rasp,  whose  power  is  enor- 
mously augmented  by  the  great  weight  of  ice  above.  Thus  in  its 
moving  course  the  glacier  is  constantly  grinding  away  the  rock-bed 
on  which  it  rests.  This  grinding  occurs  not  only  over  the  bottom 


140  tEXT-BOOK   OF   GEOLOGY 

of  the  bed,  but  along  the  sides  of  the  glacier-filled  valley  as  well, 
making  a  glacier,  therefore,  differ  very  markedly  in  erosion  work 
from  a  river,  as  we  shall  presently  see.  The  effectiveness  of  this 
engine  of  erosion  depends  largely  on  the  rigidity  with  which  the 
rocks  and  gravel,  the  teeth  of  the  rasp,  are  held  by  the  ice,  and  thus 
on  the  temperature ;  at  the  lower  end  when  the  ice  is  soft  and  melt- 
ing it  has  been  observed  pushing  over  morainal  material  without 
disturbing  it  and  this  has  sometimes  given  a  wrong  impression  that 
glaciers  are  not  very  efficacious  agents  of  erosion. 

Glaciation.  —  The  result  of  this  work  is  seen  on  bed-rock,  which 
is  smoothed,  rounded  and  polished  as  it  is  ground  away>  as  in  Fig. 


Fig.  110.  —  Glaciated  country  rock,  Eastport,  Maine. 

110,  and  scored  with  scratches  and  grooves,  called  glacial  strice 
running  in  the  direction  of  glacial  flow.  See  Fig.  111.  These  are 
made  by  pebbles,  sand,  etc.,  held  firmly  frozen  in  the  ice.  They  may 
be  very  fine  scratches  or  attain  the  dimensions  of  small  channels, 
sometimes  beautifully  fluted.  Projecting  masses  or  hummocks  of 
bed-rock,  instead  of  showing  angular,  broken  outcrops,  are  more  or 
less  smooth  and  rounded,  often  shaped  like  the  half  of  an  egg, 
especially  on  the  upstream  side,  and  these  features  are  called  roches 
moutonnees,  a  term  adopted  from  the  Swiss  mountaineers  in  allu- 
sion to  a  fancied  resemblance  to  the  backs  of  a  flock  of  sheep. 
Small  lakes  or  pools  may  occupy  rock-basins  where  depressions 
have  been  ground  out  of  bed-rock.  The  process  of  producing  such 
characteristic  features  is  called  glaciation,  and  a  country  which 
exhibits  them  is  said  to  have  been  glaciated;  they  are  seen  when 
glaciers  have  retreated,  or  disappeared,  and  by  their  presence  we 
are  able  to  recognize  the  former  existence  of  such  glaciers,  in  regions 
where  these  have  long  since  vanished,  as  in  New  England,  for 
example. 


ICE  AS  A  GEOLOGICAL  AGENT 


141 


Fig.   111.  —  Glacial  striae;    scratches  and  groovings  made  by  moving  ice  on  lime- 
stone bed-rock.     Near  Rochester,  N.  Y. 

Glacial  Valleys.  —  The  effects  of  glacial  erosion  are  seen,  not 
only  in  the  smaller  details  mentioned  above,  but  also  in  larger  fea- 
tures, such  as  affect  the  topog- 
raphy of  valleys.  The  normal 
shape  of  a  river  valley  produced 
by  corrasion  and  weathering,  has 
been  discussed  on  page  49  and 
shown  to  be  that  of  a  V  in  sec- 
tion. In  a  valley  more  or  less 
filled  with  ice  the  longitudinal 
erosion  takes  place,  not  only  on 
the  bottom,  but  also  along  the 
sides,  and  hence  a  well  glaciated 
valley  has  normally  a  U  shape 
in  cross  section,  as  seen  in  Fig. 
112.  In  a  river  valley  the  tribu- 
taries, and  the  ravines  they  have  made,  normally  join  the  main 
stream  and  its  valley  bottom  at  grade;  in  a  glaciated  valley  the 
tributary  glaciers  cannot  cut  as  fast  as  the  main  one,  and  the 
mouths  of  their  valleys  are  ground  back,  and  end  high  up  on  the 
wall  of  the  main  valley  into  which  they  discharge  in  cas- 
cades. Such  hanging  valleys,  as  they  are  called,  with  their 


Fig.  112.  —  Characteristic  U  shape  of  gla- 
cial valley.  Kern  Valley,  Cal.  H.  Gan- 
nett, U.  S.  Geol.  Surv. 


142 


TEXT-BOOK   OF   GEOLOGY 


water  falls  are  common  features 
in  the  scenery  of  the  northern 
Rocky  Mountains,  in  Switzer- 
land, and  in  Norway,  and  are 
illustrated  in  Fig.  113. 

In  a  river  valley  the  spurs  be- 
tween ravines  run  down  and  die 
out  at,  or  near,  the  river;  in  a 
glaciated  valley,  these  are 
ground  away  by  the  longitudinal 
erosion  up  to  the  level  of  the  ice 
and  after  its  recession  terminate 
in  more  or  less  well  defined  in- 
verted V  shapes  in  the  wall  of 
the  main  valley ;  these  spurs  are 
said  to  be  facetted.  Thus  U- 
shaped  sections,  hanging  tribu- 
tary valleys,  and  facetted  spurs 

are  characteristic  features  of  the  topography  of  glacial  valleys. 

The  change  of  a  normal  river  valley  into  a  glaciated  one  exhibiting 

them,  by  ice  invasion  and  retreat,  due  to  climatic  changes,  is  seen 

in  Figs.  114, 115  and  116. 


Fig.  113.  —  A  hanging  valley  and  falls. 
Yoho  Valley,  British  Columbia. 


Fig.  114. — A  mountain  mass,  normally  eroded  by  weathering  and  running  water 
and  unaffected  by  glacial  action.  The  valleys  and  ravines  are  V-shaped  in  section. 
W.  M.  Davis. 

Glaciation  by  Ice-caps.  —  In  the  great  continental  ice-caps  the 
ice  appears  to  move  en  masse  over  broad  areas  away  from  the  gen- 
eral center  of  dispersion  and  regardless  of  the  minor  features  of 


ICE  AS  A  GEOLOGICAL   AGENT 


143 


Fig.  115.  —  The  same  mass  as  in  Fig.  114  strongly  affected  by  glaciers  which  occupy 
the  valleys.  The  rugged  topography  above  the  ice,  produced  by  weathering  and 
frost  should  be  noted.  W.  M.  Davis. 


Fig.  116.  —  The  same  mass  of  mountains  as  in  Fig.  115  after  the  retreat  and  melting 
of  the  ice.  Note  the  nature  of  the  topography,  the  trough-like  form  of  the  glaci- 
ated valleys,  the  amphitheaters,  some  with  lakes,  the  hanging  valleys,  and  the 
facetted  spurs.  W.  M.  Davis. 


144  TEXT-BOOK    OF   GEOLOGY 

underlying  topography.  Hence  on  their  retreat  we  find  the  glacial 
striae,  which  indicate  the  direction  of  flow,  pointing  in  the  same  way 
over  wide  areas,  and  down,  up,  and  across  valleys.  However,  it  has 
been  noticed  that  valleys  running  somewhat  in  the  general  trend  of 
flowage,  have  in  places  exercised  some  control  and  given  rise  to 
local  sub-currents  in  the  ice.  Where  the  ice-caps  end  in  mountain 
ranges  and  push  down  their  valleys  in  projecting  tongues  to  the 
lowlands,  or  the  sea,  they  exhibit  the  erosional  features  common  to 
valley  glaciers.  Striated  and  polished  bed-rock,  roches  moutonnees, 
and  rock-basin  lakelets  over  wide  stretches  of  country  are  the 
characteristic  features  of  glaciation  by  the  continental  ice  sheets. 

The  manner  in  which  the  Antarctic  ice-cap  pushes  out  to  sea  in  the  Great 
Ice  Barrier,  see  page  128,  naturally  suggests  that  the  work  of  glaciation  must 
be  going  on  beneath  it  for  some  distance  below  sea-level,  until  the  point  is 
reached  where  it  must  tend  to  float,  and  lose  its  effectiveness  as  an  engine  of 
erosion.  The  valleys  leading  seaward  of  the  land  buried  under  the  ice  must 
be  ground  out  into  glacial  troughs,  and  eroded  below  sea-level.  If  the  ice- 
cap should  disappear,  such  "  over-deepened"  valleys  would  be  filled  with  sea 
water  and  would  form  fiords  (see  page  104).  Fiords,  then,  are  a  natural  result 
of  heavy  glaciation,  such  as  that  produced  by  ice-caps,  along  coast-lines,  or 
by  powerful  glaciers  in  high  latitudes  descending  from  the  mountains  into 
the  sea.  Unlike  ordinary  estuaries  they  are  commonly  limited  seaward  by  a 
rocky  threshold  covered  with  shallow  water  and  within  this  the  water 
deepens  sometimes  to  several  thousand  feet  (Norway),  before  it  again  shal- 
lows inland.  Such  thresholds  must  mark  then  the  limit  of  effective  down- 
ward glacial  erosion  of  the  pre-glacial  valleys. 

Glacial  Transportation.  —  There  are  no  special  problems  con- 
nected with  transport  by  ice,  as  compared  with  water.  Whatever 
lies  upon  it,  or  is  enveloped  in  it,  regardless  of  size,  is  irresistibly 
borne  along,  and  eventually  deposited  when  the  ice  melts. 

Glacial  Deposits 

Moraines.  —  The  manner  in  which  these  are  formed  has  been 
already  described.  As  seen  at  the  terminus  of  a  glacier,  or  after  its 
retreat,  they  consist  of  mingled  heaps  of  earth  and  stones,  some- 
times several  hundred,  or,  in  exceptional  cases,  1000  feet  high,  or 
even  more,  and  wide  in  proportion.  Unlike  water-laid  material, 
which  is  nicely  assorted  as  to  size  and  deposited  in  stratified  layers, 
they  consist  of  confused  debris  of  all  sizes  tumbled  together,  as 
illustrated  in  Fig.  117,  and  this  want  of  stratification  is  one  of  their 
distinguishing  characteristics.  Those  pieces  of  stone  which  have 
been  transported  as  sub-glacial  material  and  have  taken  part  in  the 
erosive  work  of  the  glacier,  have  smooth,  flat  surfaces,  or  facets, 


ICE  AS  A  GEOLOGICAL  AGENT  145 

ground  upon  them,  and  are  polished  and  striated,  or  scratched.  In 
other  words,  they  are  glaciated,  and  such  facetted,  glaciated  pebbles 
are  characteristic  features  of  glacial  work,  and  of  moraines. 
On  the  other  hand,  the  pieces  of  rock  brought  down  on,  or  in,  the 
ice,  not  being  subjected  to  grinding,  are  as  rough  and  angular  as 
when  removed  from  the  valley  walls,  and  like  those  of  any  talus. 
The  heterogeneous  material  forming  the  moraines  is  known  as 
glacial  till,  or  bowlder  clay. 

Since  at  its  lower  end,  the  ice  moves  not  only  down,  but  radially  outward, 
as  illustrated  in  Fig.  104,  the  terminal  part  of  the  lateral  moraines  may  be  so 
greatly  increased  by  this  movement  that  they  may  become  of  huge  size;  as 
the  glacier  retreats  they  continue  to  be  left  behind,  or  to  grow,  following  after 
it.  Thus  they  may  be  very  large,  while  the  terminal  portion  in  front  of  the 
ice  may  be  relatively  small,  or  almost  wanting.  They  are  really  laterally 
terminal  moraines. 


Fig.  117.  —  Glacial  till  or  bowlder  clay,  consisting  of  the  unassorted  material  of  the 
moraine.     The  Caucasus. 

Glacial  Bowlders  or  Erratics.  —  Another  characteristic  feature 
of  a  country  which  has  been  glaciated  is  the  presence  upon  it  of  scat- 
tered bowlders  of  all  sizes  and  shapes,  which  are  different  in  nature 
from  the  underlying  bed-rock.  They  are  transported  blocks  of  rock 
which  have  been  left  from  the  melting  of  the  ice;  that  they  have 
been  transported  is  proved  by  the  difference  between  them  and  bed- 
rock. They  are  called  erratics  or  glacial  bowlders.  They  are 
not  infrequently  of  great  size,  as  large  as  a  small  house,  as  illus- 
trated in  Fig.  118,  In  some  cases,  through  the  gentle  lowering  by 


146  TEXT-BOOK   OF   GEOLOGY 

the  melting  of  the  ice,  they  have  been  deposited  in  very  insecure 
positions,  and  are  known  as  perched  blocks;  they  may  be  even  so 
nicely  poised  that  although  of  large  size  they  may  be  rocked  by 
pressure  of  the  hand,  and  are  known  as  rocking  stones.  Such  de- 
position would  be  impossible  by  water  and,  in  general,  indicates 
the  agency  of  ice,  though  they  may  sometimes  be  bowlders  of  disin- 
tegration, page  30. 

Through  peculiarities  in  the  character  of  the  stone,  such  erratic  blocks  have 
often  been  traced  many  miles,  5-100,  or  even  more,  to  their  parent  ledges  of 
bed-rock.  The  most  striking  instances  of  transported  bowlders  are  those  left 


Fig.    118.  —  Glacial  erratic,     a  transported   bowlder  of  trap  resting  on  sandstone; 
weight  about  500  tons.     New  Haven,  Conn. 

by  former  ice-caps.  In  general  the  northern  parts  of  North  America  and 
Europe  are  more  or  less  covered  with  them.  If  they  can  be  traced  to  the 
parent  ledges,  as  in  the  case  of  the  "bowlder  train"  of  Richmond,  Mass.,  they 
afford  valuable  indications  of  the  direction  of  flowage,  and  confirm  the  indi- 
cations left  by  the  striae  on  bed-rock. 

Glacial  Lakes.  —  In  a  country  which  has  been  recently  glaciated, 
lakes  are  a  common  feature.  The  formation  of  small  rock-basin 
lakes  by  irregular  erosion  of  bed-rock  by  glaciers  has  been  men- 
tioned already.  But  a  much  more  important  way  in  which  glaciers 
form  lakes  is  by  moraines  left  athwart  valleys,  which  make  dams, 
ponding  back  the  drainage.  Of  the  many  beautiful  lakes  which 
lend  charm  to  the  scenery  of  the  hilly  and  mountainous  regions  of 
North  America  and  Europe,  in  the  Adirondacks,  in  New  Hamp- 
shire, Maine,  Canada,  the  Alps  and  Norway,  by  far  the  greater  part 
have  been  made  in  this  way.  See  Fig.  119.  These  dams  have  mostly 


ICE  AS  A  GEOLOGICAL  AGENT 


147 


been  left  in  valleys  on  the  retreat  of  the  continental  ice-caps.  In 
the  Southern  States,  which  have  been  unvisited'by  the  ice,  lakes 
are  rare,  or  wanting,  as  previously  mentioned,  page  81. 

In  some  of  the  Northwestern  States  and  Canada,  as  in  Minnesota 
for  example,  the  many  small  lakes  are  due,  not  to  the  damming  of 
definite  valleys,  but  to  the  filling  with  water  of  depressions  in  the 
irregular  hummocky  ridgy  surface  of  the  wide  sheet  of  morainal 
debris  left  on  the  retreat  of  the  ice-cap. 


Fig.  119.  —  Lake  due  to  glacial  action.     The  topography  in  the  background  shows 
the  characteristic  forms  and  surfaces  of  glaciation.     Sierra  Nevada,  Cal. 

Thus  glacial  lakes  are  due  to  rock  basins,  to  morainal  dams  in  val- 
leys, to  hollows  in  morainal  deposits,  and  to  the  filling  with  water  of 
the  kettles  mentioned  beyon,d. 

Ice-Cap  Deposits;  Glacial  Drift.  —  The  deposits  left  by  the  con- 
tinental ice  sheets,  or  caps,  are  somewhat  different  from  those  of 
valley  glaciers.  As  they  have  no  side  boundaries  there  are  no  dis- 
tinct lateral  moraines,  but  only  the  terminal  one.  The  former  edge 
of  the  ice  is  marked  by  this  terminal  moraine,  a  deposit  of  till  cross- 
ing the  country  as  a  series  of  hills,  hummocks,  knobs,  and  ridges, 
with  depressions  between,  called  "kettles"  which  are  often  filled  with 
water.  As  the  ice  continues  to  retreat  by  melting,  the  material  it 
contains  is  left  as  a  broad  sheet  of  till,  or  bowlder  clay,  covering 
the  country.  As  the  retreat  apparently  takes  place  irregularly,  with 
oscillations  of  advance  and  recession,  like  those  observed  in  modern 
glaciers,  such  stages  are  marked  by  new  terminal  moraines,  which. 


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as  they  lie  back  of  the  most  advanced  one,  have  been  called  reces- 
sional. Certain  peculiar  forms  of  these  glacial  deposits  have  been 
called  drumlins;  they  consist  of  rounded,  elongated  hills,  or  short 
ridges,  of  unassorted  till,  whose  longer  axis  points  in  the  direction  of 
ice  movement ;  it  is  not  definitely  known  how  the  deposits  assumed 
this  form  and  position.  They  are  especially  common  in  central 
New  York  and  in  eastern  Wisconsin,  and  are  illustrated  in  Fig.  120. 
All  the  material,  —  the  sheets  of  till,  the  moraines,  erratic  blocks, 


i 


Fig.  120.  —  Drumlin  near  Newark,  N.  Y.     G.  K.  Gilbert,  U.  S.  Geol.  Surv. 

etc.,  —  whose  deposit  on  glaciated  bed-rock,  whose  want  of  gradual 
transition  into  country  rock,  observed  where  soil  is  found  in  place, 
and  whose  foreign  nature  prove  it  to  have  been  transported,  and 
whose  heterogeneous,  unassorted  character  and  glaciated  pebbles 
show  it  to  have  been  deposited  by  ice,  is  known  under  the  general 
term  of  the  glacial  drift.  This  term  is  also  applied  to  these  materials 
where  they  have  been  transported  or  washed  and  laid  down  by 
water ;  in  this  case  stratification  is  more  or  less  distinctly  shown  in 
the  deposits  and  they  are  commonly  referred  to  as  "modified"  or 
"stratified"  drift.  These  we  will  now  consider. 

Fluvio-Glacial  Deposits.  —  The  melting  of  the  ice  of  glaciers  and 
the  natural  drainage  of  the  valleys  occupied  by  them  produce 
streams  heavily  charged  with  sediment,  as  described  on  page  136. 
The  streams,  unable  to  carry  this  material,  deposit  it  on  the  valley 
floors,  building  up  flood-plains,  in  which  they  wander  in  devious  and 
shifting  channels,  and  this  deposit,  sloping  downward  along  the  val- 
ley from  the  terminal  moraine,  is  known  as  the  valley  train.  In  the 


ICE  AS  A  GEOLOGICAL  AGENT 


149 


case  of  continental  ice-caps  the  washed-down  material,  instead  of 
being  concentrated,  as  in  a  valley  train,  may  be  spread  widely  by 
meandering  streams  over  broad  areas,  giving  rise  to  what  are  known 
as  outwash  plains,  or  frontal  aprons.  Such  deposits,  since  they  are 
laid  by  water,  are  more  or  less  distinctly  stratified,  while  the  peb- 
bles of  the  gravels  may  become  rounded  through  attrition,  and 
lose  more  or  less  completely  the  facets  and  scratches  they  exhibit 
in  the  moraines.  Fig.  121  shows  a  section  of  this  water-laid  drift 
resting  on  unassorted  till. 

In  the  outwash  plains,  conical  depressions,  sometimes  100  feet  deep,  are 
found  which  are  called  kettles.  They  are  well  illustrated  in  the  sand  plain 
above  New  Haven,  Conn.  They  are  supposed  to  be  cavities  made  by  the 
melting  of  isolated  blocks  of  ice,  left  for  a  time  by  the  retreat  of  the  irregu- 
lar glacial  front,  and  then  surrounded  and  more  or  less  covered  by  sediment 
from  the  glacial  streams. 


Fig.  121.  —  Water-laid  glacial  drift  on  unassorted  till.     Columbus,  Ohio. 

Kames  and  Eskers.  —  These  are  peculiar  forms  of  deposit  made 
by  the  sediment-laden  streams  from  the  ice-caps.  Kames  are  hills, 
knobs,  or  short  ridges,  sometimes  attaining  a  height  of  100  feet, 
which  resemble  drumlins,  but  differ  from  them  in  that  they  consist 
of  stratified  material  and,  instead  of  pointing  in  the  direction  of  ice 
flow,  they  tend  to  arrange  themselves  athwart  it.  They  are  apt  to 
occur  in  groups  with  depressions  between  which  sometimes  contain 
water.  They  are  thought  to  represent  crevasses,  or  other  openings, 
or  depressions  in  the  irregular  front  of  the  ice  sheet,  which  have 
been  filled  with  sediments  and  left  as  projections  upon  its  melting. 

Eskers  are  long  winding  ridges  of  stratified  sands  and  gravel,  10, 
20,  or  even  100  feet  high,  with  very  narrow  crests  and  trending  in 


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the  general  direction  of  ice  flow.  They  may  strikingly  resemble 
artificial  railway  embankments.  While  found  in  various  parts  of 
the  Northern  States,  they  are  particularly  striking  in  Maine.  They 
have  a  great  development  in  Scandinavia  where  they  run  across 
country,  in  some  cases,  for  many  miles.  They  are  illustrated  in  Fig. 
122.  It  is  supposed  that  they  have  been  built  by  streams  which 
had  cut  channels  for  themselves  on,  in,  and  under  the  ice.  The  de- 
posit would  be  confined  by  the  ice  walls,  and,  upon  the  melting  and 
retreat  of  the  ice  sheet,  would  be  left  as  a  sinuous  ridge,  marking  the 
former  channel. 


Fig.   122.  —  The  esker  of  Punkaharju,  Puruvesi,  Finland.     In  Scandinavia  such  a 
ridge  is  called  an  'ose,'  plural  'osar.' 

With  respect  to  the  deposits  made  and  left  by  ice-caps  the  following  table 
will  enable  the  student  to  summarize  the  main  features  brought  out  in  the 
above  discussion. 

Ice-laid,  heaped 

Moraines.    Irregular  ridges,  when  terminal,  transverse  to  ice  flow. 
Drumlins.    Ovate  hills,  elongate  parallel  to  ice  flow. 

Water-laid,  stratified 

Kames.    Round  to  ovate  hills,  grouped  transverse  to  ice  flow. 
Eskers.    Winding,  very  elongate  ridges  often  parallel  to  ice  flow. 
Frontal  aprons.    Outwash  plains,  beyond  morainal  deposits. 


ICE  AS  A  GEOLOGICAL  AGENT 


151 


Icebergs 

Icebergs  are  formed  in  northern  regions  where  glaciers  come 
down  from  the  land  and  enter  the  sea.  Pushing  out  into  the  water, 
the  buoyancy  of  the  ice  floats  the  end  of  the  glacier  and  causes 
huge  masses  to  break  away,  which  float  off  as  icebergs.  The  process 
is  illustrated  in  Fig.  123. 


Fig.  123.  —  Diagram  illustrating  the  formation  of  icebergs. 

Icebergs  are  sometimes  of  very  great  dimensions,  rising  200  feet  or  more 
above  the  sea,  though  this  is  uncommon.    The  specific  gravity  of  solid  ice  is 


Fig.  124.  —  Icebergs  in  the  Polar  sea.  Only  a  small  part  of  the  berg  is  above  water, 
but  this  may  be  the  longest,  thinnest  part  of  the  mass  which  projects,  like  the 
apex  of  a  floating  cone.  It  is  surrounded  by  floes  of  frozen  sea-water. 

about  0.9,  compared  with  water,  but  glacier  ice  is  more  or  less  porous,  and  per- 
haps as  much  as  one-seventh  of  the  mass  of  a  berg  may  be  above  water.  Thus 
they  may  extend  downward  1500  feet;  they  have  been  observed  aground  in 
water  of  this  depth.  The  volume  of  some  bergs  has  been  estimated  to  be  as 


152  TEXT-BOOK   OF   GEOLOGY 

much  as  500,000,000  cubic  yards,  which  would  cover  an  area  of  one  mile  square 
500  feet  deep.  The  largest  bergs  are  those  which  break  from  the  great  ice  bar- 
rier surrounding  the  Antarctic  continent  (see  page  128) ;  they  are  often  remark- 
ably tabular  in  form.  The  source  of  the  North  Atlantic  bergs  is  chiefly  the 
great  ice-cap  of  Greenland,  especially  that  prolongation  of  it  into  the  sea 
known  as  the  Humboldt  glacier,  which  presents  an  ice-front,  or  cliff,  60  miles 
long.  It  should  be  remembered  that  icebergs  are  always  composed  of  fresh- 
water ice  and  are  made  on  the  land  by  glaciers;  the  ice  of  the  frozen  sea  it- 
self forms  ice-fields,  called  floes,  which  are  not  originally  more  than  8-10  feet 
thick,  though  by  pressure,  crowding,  and  over-riding,  they  may  in  places  be- 
come several  times  this  thickness. 

By  the  general  circulation  of  oceanic  waters,  which  has  been 
previously  described,  all  the  floating  ice  of  high  latitudes  in  both 
hemispheres  is  gradually  drifted  into  warmer  seas,  northward  and 
southward  respectively  towards  the  equator,  and  eventually  melted. 
In  the  North  Atlantic  the  bergs  may  be  floated  as  far  south  as  40°, 
making  a  formidable  menace  to  navigation.  Were  it  not  for  this 
general  law,  ice  would  accumulate  indefinitely  in  polar  regions,  until 
the  greater  part  of  the  waters  of  the  world  were  locked  up  in  polar 
ice-caps,  while  the  lands  would  be  arid  deserts. 

Geological  Work  of  Floating  Ice.  —  In  high  latitudes  the  float- 
ing ice  of  the  bergs  and  floes,  driven  against  the  shore  by  winds  and 
tidal  currents,  chafes  against  the  rocks,  eroding  and  polishing  them. 
Icebergs  grinding  on  the  bottom  may  also  erode  and  scratch  the 
rocks,  but  such  effects  are  probably  relatively  small  in  amount. 
Icebergs,  like  other  glacial  ice,  may  contain  englacial  material/earth 
and  bowlders  frozen  in  the  mass,  which  is  dropped  on  the  bottom  as 
the  berg  melts.  Although  locally  such  deposits  must  be  slight,  in 
sum  total  the  amount  of  material  transported  in  this  way  must  be 
large.  Since  the  North  Atlantic  bergs  are  apt  to  go  aground  in 
the  shallow  water  southeast  of  Newfoundland  and  melt,  it  has  been 
supposed  that  the  Grand  Banks  have  been  largely  formed  in  this 
way,  but  there  is  no  direct  evidence  of  this,  and  a  rough  estimate  of 
the  quantities  of  material,  the  number  of  bergs,  and  the  time  re- 
quired to  form  them  makes  it  extremely  improbable. 


CHAPTER  VI 
UNDERGROUND  WATER 

The  various  agencies  which  have  been  described  as  operating 
upon  the  surface  of  the  earth,  that  is,  the  atmosphere  and  the  watery 
envelope  of  the  globe  in  its  varied  forms  of  running  water,  the  ocean, 
lakes,  and  snow  and  ice,  all  tend  to  cut  away  its  irregularities  and  to 
furnish  material  which  is  used  in  filling  up  depressions.  Hence  they 
tend  in  the  long  course  of  time  to  make  its  surface  smoother,  and  are 
therefore  spoken  of  as  levelling  agencies.  But  water,  in  addition  to 
this  work  of  levelling,  which  is  largely  mechanical  in  its  nature,  per- 
forms other  geological  work  of  vast  importance,  and  chiefly  in  a 
chemical  way.  One  phase  of  this  chemical  work  has  been  already 
discussed  in  connection  with  the  formation  of  soil,  page  23,  and 
another  has  been  alluded  to  in  relation  to  that  part  of  a  river's  bur- 
den which  is  carried  in  solution.  But  the  matter  as  a  whole  is  best 
understood  through  a  discussion  of  the  nature  of  ground-water  and 
the  functions  it  performs. 

Ground-water.  —  It  has  been  previously  stated  that  of  the  water 
that  falls  in  rain  one  part  is  evaporated  and  goes  back  into  the  air, 
that  another  portion  passes  directly  over  the  surface  into  the  sea, 
while  a  third  portion  sinks  into  the  soil,  and  into  the  cracked  and 
broken  bed-rock  below  it.  It  is  this  last  part  of  the  rainfall,  which 
thus  sinks  downward,  that  we  know  as  ground-water.  Various 
things  may  happen  to  it;  it  may  find  its  way  to  the  surface  as 
springs  and  aid  in  the  general  run-off;  it  may  be  drawn  to  the  sur- 
face by  capillary  attraction  through  the  pores  in  the  soil  and  be 
evaporated ;  it  may  be  sucked  up  by  plants  and  evaporated  through 
their  leaves;  it  may  never  return  to  the  surface,  but  find  its  way 
into  the  sea  by  underground  channels;  it  may  remain  held,  for 
aught  we  may  know,  for  indefinite  periods  in  deep  fissures  in  the 
rocks ;  and  lastly  it  may  enter  into  chemical  combinations  with  the 
minerals  of  the  rocks  and  become  fixed.  Before  studying  the  geo- 
logical side  of  these  happenings  certain  features  of  ground-water 
should  be  explained. 

Situation  of  Ground-water.  —  The  water  that  percolates  down- 
ward fills  the  fissures  in  the  rocks  and  the  interspaces,  or  pores,  be- 
tween the  grains  of  overlying  soil  up  to  a  certain  level.  Above 

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this  level  the  soil  may  be  moist,  but  the  pores  are  not  filled  ;  below 
it  the  soil  is  saturated,  consisting  of  a  mixture  of  soil  grains  and 
water,  like  sand  and  water  in  a  basin,  and  thus  forming,  so  to  speak, 
an  underground  lake.  The  surface,  or  upper  level,  of  this  ground- 
water  is  called  the  water-table  and  it  is  this  water-table  that  one 
endeavors  to  reach  and  penetrate  in  ordinary  dug  wells.  The  depth 
of  the  water-table  below  the  surface  of  the  ground  varies  in  a  given 
locality  in  response  to  wet  or  dry  seasons;  in  different  localities 
according  to  circumstances,  especially  the  annual  rainfall.  Thus 
in  very  humid  regions  it  is  generally  but  a  few  feet  below  the  sur- 
face, while  in  very  arid  regions  it  may  be  hundreds  of  feet  down, 
entirely  below  the  soil,  and  in  the  rocks  beneath.  The  depth  to 
which  ground-water  may  penetrate  in  the  rocks  is  unknown,  but 
experience  in  deep  mines  seems  to  show  that  it  is  not  very  far,  and 


Fig.  125.  —  Ideal  section  across  a  river  valley,  showing  position  of  ground-water,  its 
relation  to  bed-rock  below,  and  the  contour  of  the  water-table  with  reference  to 
that  of  the  ground  above.  Vertical  scale  exaggerated.  After  Slichter. 

it  must  in  any  case  cease  at  the  point  where,  through  the  pressure 
of  the  superincumbent  masses  above,  cracks  and  fissures  in  the  un- 
derlying rock  can  no  longer  exist.  This  occurs,  however,  at  a  num- 
ber of  miles  below  the  surface,  since  it  has  been  demonstrated  by 
Adams  that  cavities  can  remain  open  to  a  depth  of  at  least  11  miles. 
Beyond  this  it  may  be  that,  in  response  to  the  pressure,  all  rocks, 
however  rigid  they  may  appear  at  the  surface,  are  weaker  than  the 
pressure  upon  them  and,  yielding  like  metals  under  the  stamping  of 
a  die,  all  cavities  in  them  may  be  closed  up. 

Contour  of  the  Water-table.  —  The  contour  of  the  water-table 
in  a  general  way  follows  that  of  the  ground  above,  rising  under  hills 
and  sinking  in  valleys,  as  illustrated  in  Fig.  125.  If  the  surface  of 
the  ground  intercepts  the  water-table  then  the  ground- water  appears, 
and  if  this  occurs  on  a  hillside  a  spring  results,  while,  if  the  surface 
of  the  ground  and  the  water-table  coincide  for  a  distance,  this  area 
is  a  swamp  or  bog.  In  lakes  and  rivers  the  contour  of  the  ground 
sinks  below  that  of  the  water-table  and  the  latter  stands  revealed. 


UNDERGROUND   WATER  155 

Porosity  of  Soil  and  Rocks.  —  The  volume  of  space  between  the 
grains  of  a  soil,  or  its  porosity,  which  can  be  occupied  by  water,  de- 
pends very  much  on  the  nature  and  arrangement  of  the  particles 
composing  it.  In  ordinary  sand  the  volume  of  pore  space  is  usually 
about  30  per  cent,  and  may  be  considerably  larger;  in  ordinary 
loam,  which  contains  a  good  deal  of  clay,  see  page  29,  it  may  be 
still  larger,  from  40-50  per  cent.  Thus  in  a  natural  basin  covering  a 
certain  area,  in  which  the  average  depth  of  sandy  soil  is  30  feet  to 
bed-rock,  and  in  which  the  water-table  stands  15  feet  below  the 
surface,  the  total  volume  of  ground-water  would  equal  that  of  a  lake 
covering  the  area,  and  about  5  feet  deep. 

All  rocks  are  in  some  degree  porous,  sandstone  the  most  so,  the 
volume  of  interspaces  rising  in  some  cases  to  30  per  cent  in  this 
rock,  while  in  crystalline  rocks,  like  granite  for  example,  it  may  be 
only  one  per  cent,  or  even  less.  This  applies  to  the  interspaces  be- 
tween the  rock-grains  and  not,  of  course,  to  cracks  or  fissures,  which 
can  only  contain  a  small  part  of  ground-water.  The  average  pore 
space  is  probably  not  over  10  per  cent.  Assuming  this  amount  for 
the  surface,  and  that  it  diminishes  with  the  depth,  it  was  formerly 
calculated  that  the  total  quantity  of  water  held  underground  in 
the  world  was  as  much  as  one-sixth  that  contained  in  the  ocean, 
but  later  investigations,  which  show  the  dry  nature  of  the  rocks  in 
deep  mines,  have  led  to  calculations  which  very  greatly  reduce 
this  amount.  In  any  case  the  total  amount  is  actually  very  great. 

Motions  of  Ground-water.  —  Probably  only  the  lower  depths  of 
ground-water  remain  stationary  for  any  length  of  time ;  that  occupy- 
ing the  upper  layers  of  rock  and  the  soil  has  a  slow  but  regular 
motion,  depending  on  the  difference  of  pressure  due  to  gravity,  from 
point  to  point.  Thus  it  urges  its  way  slowly  onward  from  higher  to 
lower  levels  and,  ultimately,  like  the  water  of  the  run-off,  it  seeks 
its  goal  in  the  sea.  In  the  sands  filling  a  valley  below  the  bed  of  a 
river,  as  illustrated  in  Fig.  125,  the  water  underground  is  following 
the  same  course  as  that  filling  the  channel  above,  though  at  a  vastly 
slower  rate.  This  is  known  as  the  under-flow.  The  rate  of  move- 
ment is  relatively  rapid  in  gravels  and  coarse  sands,  much  slower 
through  finer  sands,  while  in  fine  clays  it  is  almost  indefinitely 
slow.  A  familiar  example  of  this  is  seen  in  the  rapidity  with  which 
puddles  in  a  sandy  road,  after  a  shower,  drain  away,  whereas  in  a 
bed  of  clay  they  remain  until  evaporated.  Thus  clay,  and  rocks 
composed  of  clay  (shale),  are  practically  impervious  to  the  move- 
ment of  water,  while  in  sand  and  sandstone,  on  account  of  the 
perviousness,  it  much  more  readily  takes  place.  Yet  even  in  sands  the 


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TEXT-BOOK    OF   GEOLOGY 


movement  is  very  slow,  and  a  general  average  of  the  rate  of  under- 
flow, as  determined  from  experiments,  is  about  one  mile  per  year. 
The  natural  line  of  drainage  over  the  underground  surface  of  bed- 
rock, along  which  the  underflow  takes  place,  may  be  called  the 
underground  thalweg  (German,  valley  way),  and  is  illustrated  in 
Fig.  125. 


Fig.  126.  —  Diagrams  illustrating  conditions  favorable  for  ordinary  hillside  springs. 
A,  in  section;  B  shows  how  the  contour  of  the  thalweg  concentrates  the  seepage  to 
form  a  spring.  Dotted  line,  high-water  stage,  spring  at  a;  solid  line,  low-water 
stage  and  spring  at  6. 


Fig.   127.  —  Thousand  Springs,   Snake  River  canyon,   Idaho.     I.   C.  Russell,   U.  S. 

Geol.  Surv. 

Springs.  —  Where  the  contour  of  the  ground  intercepts  the  water- 
table,  ground-water  appears  at  the  surface.  A  general  oozing  out 
of  the  water  under  these  conditions  is  known  as  seepage,  but  if  the 
circumstances  are  such  that  a  volume  of  the  water  issues  out  in 
quantity  sufficient  to  form  a  distinct  current,  an  ordinary  spring 
results,  as  illustrated  in  Fig.  126,  A  and  B.  Fig.  127  illustrates 
the  cutting  of  the  water-table  by  the  side  of  a  canyon,  and  the 
issuance  of  the  underground  water  in  a  series  of  springs. 


UNDERGROUND   WATER 


157 


Another  type  of  spring  is  formed  when  the  surface  water  enters 
and  fills  some  porous,  inclined  layer,  such  as  one  of  sand,  or  sand- 
stone, lying  between  two  impervious  ones,  as  of  clay  or  shale.  Driven 
onward  by  the  weight  of  the  column  behind  it  the  water  may  ac- 


Fig.  128.  —  Section  illustrating  conditions  favorable  for  springs,  if  fissures,  such  as  /, 
are  present,  or  for  artesian  wells  if  fissures  are  absent. 

quire  such  hydrostatic  pressure  that,  if  it  encounters  the  plane  of  a 
fissure,  it  may  be  driven  up  along  this,  through  suitable  channel- 
ways,  and  issue  at  the  surface  as  a  spring.  This  is  illustrated  in 
Fig.  128.  Such  springs  are  often  called  fissure  springs. 

In  such  an  arrangement  as  is  postulated  in  the  figure,  along  the  line  of  in- 
tersection of  the  fissure  with  the  surface,  springs  might  be  expected  at  va- 
rious points  where  suitable  channels  for  the  upward  flow  of  the  water  exist, 
as  suggested  in  the  diagram. 

Such  springs  are  usually  very  steady  in  their  flow,  and  less  liable  to  be 
affected  by  droughts  than  ordinary  hillside  springs.  While  usually  cold,  the 
water  may  come  in  contact  with  heated  rocks  and  issue  in  a  warm  spring. 
This  is  probably  the  explanation  of  the  warm  springs  occurring  at  various 
places  in  the  Appalachians,  as  at  Hot  Springs,  Virginia.  Or  the  water  may 
sometimes  take  up  substances  in  solution  and  give  rise  to  mineral  springs,  as 
at  Saratoga,  New  York. 

The  condition  under  which 
porous  beds  become  filled  with 
water  is  important,  not  only  for 
fissure  springs,  but  also  for  ar- 
tesian wells,  described  below.  It 
is  illustrated  in  Fig.  129.  The 
porous  layers  BD  become  filled, 
not  alone  by  the  rain  which  may 
chance  to  fall  on  their  exposed 
surfaces,  and  by  the  water  which 

is  shed  upon  them  from  the  Fig.  129.  -  Diagram  to  illustrate  entrance  of 
i  .  i  -  ,  water  into  porous  rock  layers,  or  strata, 

higher  impervious  slopes  A  and         ,  __    .  .         u   ,       Dr> 

ACE,    impervious    beds;    BD,   porous    ones. 
C,  but  there  is  also  an  entrance        RR>  courge  of  river 

of  water  into  them  from  the  cur- 
rent of  the  river,  concentrated  from  the  water-shed  above  and  beyond,  which, 
spreading  out  in  them,  furnishes  a  constant  source  of  supply. 


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TEXT-BOOK   OF   GEOLOGY 


Artesian  Wells.  —  If  under  the  conditions  described  above  as 
producing  fissure  springs,  where  an  inclined  porous  rock-bed  be- 
tween impervious  ones  becomes  filled  with  water  under  sufficient 
pressure,  a  bore  hole  be  put  down,  the  water  will  rise  to  the  surface, 
producing  an  artesian  well.  This  may  be  regarded  as  an  artificial 
fissure  spring,  and  the  arrangement  illustrated  in  Fig.  131  is  es- 
pecially suitable  for  such  wells.  In  some  cases  the  porous  layer 
may  have  underground  the  form  of  a  basin,  but  this  is  not  a 


Fig.  130.  —  Artesian  well,  near  Provo,  Utah.    G.  E.  Richardson,  U.  S.  Geol.  Surv. 

necessary  condition.  The  height  to  which  the  water  will  rise  above 
the  surface  depends  on  the  pressure,  which  in  turn  depends  on  the 
height  of  the  water  column,  or  "head,"  in  the  porous  layer  above  the 
point  of  exit,  as  shown  in  the  figure.  An  artesian  well  is  shown  in 
Fig.  130. 


Fig.  131.  —  Section  showing  conditions  favorable  for  an  artesian  well.    Vertical  scale 

exaggerated. 

The  terms  porous  and  impervious  used  in  this  connection  are  relative  ones, 
as  all  rock-beds  are  to  some  extent  porous.    In  addition  to  the  conditions  men- 


UNDERGROUND  WATER  159 

tioned  as  necessa^  for  artesian  wells,  others  are  that  there  should  be  a 
rainfall  over  the  region  where  the  porous  layer  comes  to  the  surface  sufficient 
to  keep  it  filled  with  water,  and  also  that  the  rock-beds  should  not  be  so 
cracked,  fissured,  or  displaced  as  to  permit  the  easy  escape  of  the  water  and 
thus  cause  the  loss  of  the  required  pressure.  Artesian  wells  cannot,  therefore, 
be  made  in  any  place  by  simply  boring  deeply  enough,  unless  the  requisite 
geologic  conditions  are  present.  Any  well  bored  in  rock,  if  it  simply  in- 
tercepts the  level  of  ground-water,  is  often  called  an  artesian  well,  but  this 
is  an  incorrect  use  of  the  term;  there  is  no  difference  in  principle  between  one 
of  this  kind  and  an  ordinary  dug  well.  Some  of  the  most  important  water- 
bearing formations  in  the  United  States,  which  furnish  artesian  wells,  are 
found  in  the  so-called  Dakota  sandstone,  which  comes  to  the  surface  along 
the  Rocky  Mountains,  and  underlies  North  and  South  Dakota,  Kansas, 
Nebraska,  and  extends  into  Canada;  in  the  Saint  Peter  sandstone,  which  out- 
crops in  central  Wisconsin,  and  underlies  Illinois,  Indiana,  Iowa,  and  Ohio ; 
and  in  the  beds  of  sands  and.  clays,  which,  beginning  on  Long  Island,  extend 
southward  to  Texas,  forming  the  Atlantic  coastal  plain.  In  New  England 
the  conditions  are  unfavorable  for  artesian  wells. 

The  depth  to  which  wells  must  be  bored  in  some  places  before  artesian 
water  is  attained  is  very  great,  up  to  4000  feet,  examples  being  found  in 
Berlin,  St.  Louis,  and  Pittsburgh;  those  of  1000  feet  are  not  uncommon,  while 
along  the  Atlantic  coast  they  are  generally  shallow,  100 — 300  feet.  The 
volume  of  water  may  be  very  large,  the  great  12-inch  well  at  St.  Augustine, 
Florida,  with  a  depth  of  1400  feet,  supplying  10,000,000  gallons  a  day.  Where 
many  wells  are  put  down  close  together  they  may  interfere  with  each  other, 
and  even  lower  the  pressure  to  such  an  extent  that  the  water  will  no  longer 
overflow. 

Geologic  Work  of  Ground-water 

Water  underground  is  a  very  important  geologic  agent.  The 
chief  work  that  it  does  is  to  take  substances  into  solution  and  carry 
them  elsewhere,  often  finally  depositing  them,  and  it  is,  therefore, 
chemical  in  its  nature.  Although  this  work  may  seem  small,  when 
examined  in  detail,  the  total  results  performed  during  the  long  period 
of  geologic  time  have  been  enormous.  In  some  measure  this  work 
has  already  been  considered.  Thus  in  the  description  of  the  decay 
of  rocks  and  the  formation  of  soil  it  was  pointed  out  that  certain 
constituents,  like  the  alkalies  in  the  feldspars  in  the  rocks,  went  into 
solution  and  were  removed.  It  was  also  shown  that  calcium  car- 
bonate, a  common  rock-making  material,  under  the  influence  of 
water  and  carbon  dioxide  was  dissolved  and  carried  away  (page 
25).  These  actions  are  accomplished  by  atmospheric  water  as  it 
passes  underground,  and  may  thus  be  regarded  as  the  first  stages 
of  the  work  of  ground-water.  Again  its  work  was  in  some  degree 
considered,  when  it  was  stated  what  a  large  proportion  of  the 
burden  carried  by  rivers  consisted  of  material  in  solution  (page  45) . 


160 


TEXT-BOOK    OF   GEOLOGY 


This  shows  the  removal  of  the  substances  dissolved  by  ground- 
water  and  their  ultimate  goal  in  the  sea.  And  again  in  the  for- 
mation of  salt  lakes,  and  in  the  deposits  which  occur  in  them 
(page  84) ,  is  seen  this  work  of  solution,  removal,  and  deposit. 

But  although  all  these  illustrate  the  general  chemical  work  of 
water,  partly  on  the  surface  and  partly  underground,  there  are 
certain  features  which  demand  particular  consideration. 

Solution.  —  The  solvent  action  of  rain-water  passing  into  the  soil 
and  rocks  is  greatly  increased  by  the  substances  which  it  may  carry 


Fig.  132.  —  Rock  whose  more  soluble  parts  are  being  dissolved  by  the  action  of  at- 
mospheric waters.  Wind  aids  the  rain  in  removing  the  loosened  material.  Near 
Livingston,  Mont.  C.  D.  Walcott,  U.  S.  Geol.  Surv. 

with  it,  or  which  it  may  otherwise  obtain.  In  its  passage  through 
the  air  it  dissolves  notable  quantities  of  carbon  dioxide  and  oxygen, 
with  minute  amounts  of  other  materials,  and  is  thus  equipped  for 
doing  chemical  work.  See  Fig.  132.  In  passing  through  the  soil  of 
humid  regions  it  may  absorb  much  more  carbon  dioxide,  and  also 
organic  acids  produced  by  the  decomposition  of  vegetable  matter. 
In  many  places,  particularly  volcanic  regions,  volatile  substances, 
especially  carbon  dioxide,  are  evolved  from  the  depths,  and  may 
dissolve  in  the  water  underground  and  thus  greatly  augment  the 
amount  of  chemical  reagents  present  in  it.  All  of  these  promote  its 
efficiency  as  a  solvent.  In  addition,  as  it  passes  into  deeper  zones, 


UNDERGROUND  WATER  161 

it  may  become  subject  to  pressure,  or  come  in  contact  with  heated 
rocks  and  have  its  temperature  raised,  both  of  which  largely  in- 
crease its  chemical  activity;  the  amount  of  gases,  such  as  carbon 
dioxide,  which  it  can  dissolve  and  hold  in  solution,  is,  indeed,  pro- 
portional to  the  pressure. 

Pure  water  itself  acts  upon  many  things,  but  with  its  chemical 
efficiency  heightened  as  just  described,  it  attacks  the  mineral  sub- 
stances composing  the  rocks  and  soils;  some  of  them  it  takes  di- 
rectly into  solution,  as,  for  example,  gypsum,  CaS04  .  2H20;  with 
many  others  a  chemical  reaction  takes  place  with  formation  of  new 
compounds,  some  of  which  are  soluble  and  are  carried  away,  while 
the  insoluble  ones  remain.  This  latter  process  is  illustrated  in  the 
breaking  up  and  decay  of  feldspar,  as  described  under  the  forma- 
tion of  soil;  the  alkaline  carbonates  produced  are  leached  out, 
whereas  the  insoluble  kaolin,  or  clay,  remains. 

One  sees,  therefore,  from  this  that  the  outer  portion  of  the  earth's 
crust,  over  the  land  surfaces,  is  one  of  destruction  and  change. 
And  this  is  not  limited  to  the  merely  superficial  layer,  in  which  the 
rocks  are  changed  into  soil,  but  extends  downward  into  the  zone  of 
cracked  and  fissured  rock  far  below. 

The  material  taken  up  and  held  in  solution  may  pursue  one  of 
two  courses,  depending  on  what  happens  to  the  water  containing  it. 
It  may  work  down  deeply  into  the  rocks  and  be  there  deposited,  or 
it  may  pass  into  the  drainage  by  leaching  through  the  soil,  or  by 
coming  out  in  springs,  and  be  thus  carried  into  the  ocean.  The  first 
course  may  be  considered  a  little  later,  we  are  here  concerned  with 
the  material  taken  away. 

Chemical  Denudation.  —  The  process  by  which  the  land  surface 
is  wasted  by  material  going  into  solution  and  being  taken  into  the 
sea  is  known  as  chemical  denudation,  to  distinguish  it  from  the  me- 
chanical wear  of  ordinary  erosion.  In  the  aggregate  it  amounts  to 
an  enormous  sum  each  year.  It  has  already  been  briefly  mentioned 
in  the  discussion  of  a  river's  burden.  On  the  basis  of  a  large 
number  of  analyses  of  the  waters  of  the  Mississippi,  which  give 
the  average  percentage  of  the  salts  which  it  contains,  and  of  the 
total  volume  of  its  discharge,  Dole  and  Stabler  have  calculated  that 
each  year  there  are  removed  108  tons  (metric)  of  matter  in  solution 
per  square  mile  over  its  basin.  This  does  not  mean,  of  course,  that 
this  amount  is  taken  from  each  square  mile,  but  is  the  general 
average;  it  is  more  in  some  places,  in  others,  less.  If  we  estimate 
the  area  of  the  basin  in  round  numbers  at  1,265,000  square  miles 
this  gives  136,620,000  tons  per  annum  for  the  total  basin.  For  the 


162  TEXT-BOOK    OF   GEOLOGY 

entire  United  States  (3,088,500  square  miles)  —  regarding  denuda- 
tion of  the  Great  Basin  as  not  adding  to  the  ocean  —  the  average  is 
estimated  at  nearly  79  metric  tons  per  square  mile.  This  figure  has 
been  used  by  F.  W.  Clarke  as  a  fair  average  for  the  whole  of  North 
America,  and  gives  474,000,000  tons  for  its  total  area.  For  the  whole 
world  this  estimate  of  79  tons  would  be  too  high,  for  arid  desert 
regions,  like  those  of  central  Asia  and  Africa,  have  a  scanty  drain- 
age, and  .thus  lose  a  relatively  much  smaller  amount  of  mineral  mat- 
ter in  solution.  The  same  is  true  in  humid  tropical  countries  where 
the  soil,  held  in  place  by  dense  vegetation,  has  for  centuries  past 
been  leached  of  its  soluble  matter,  and  in  Arctic  regions  where  the 
drainage  is  largely  over  frozen  soils  which  contribute  very  little. 
Taking  these  facts,  as  well  as  others,  into  consideration,  Clarke 
estimates  that  the  average  is  about  68.4  tons  per  square  mile  for 
the  land  surface  of  the  world,  leaving  out  the  polar  areas  which 
have  little  or  no  water  drainage;  or  a  total  of  2,735,000,000  tons 
per  year.  Making  certain  corrections  we  may  deduce  from  this 
that  the  land  surface  of  the  globe,  which  is  subject  to  the  solvent 
action  of  water,  is  lowered  on  the  average  by  this  agency  one  foot 
in  30,000  years. 

Results  of  Solution  on  Carbonate  Rocks.  —  Outside  of  the  pro- 
cess of  soil  formation,  in  which  solution  also  plays  an  important 
part,  the  most  obvious  results  of  its  action  are  seen  in  the  effects  it 
has  upon  rocks  wholly,  or  partly,  composed  of  carbonates.  The 
most  important  rock-forming  carbonates  are  those  of  calcium,  mag- 
nesium, and  ferrous  iron;  CaCO3,  MgC03,  and  FeC03.  Of  these  the 
first  two,  and  especially  the  carbonate  of  lime,  are  the  most  impor- 
tant; vast  stretches  of  the  land  being  covered  with  beds  of  rock, 
hundreds  or  even  thousands  of  feet  thick,  which  are  composed  of 
them.  Such  rocks,  if  they  consist  wholly,  or  mainly,  of  carbonate 
of  lime,  are  called  limestone;  if  they  contain  much  carbonate  of 
magnesia,  dolomite.  In  addition,  beds  of  sandstone,  excepting  most 
of  those  that  are  red  or  brown,  may  contain  a  cement  of  carbonate 
of  lime  holding  together  the  grains  of  sand.  Now  since  these  car- 
bonates, and  especially  lime  carbonate,  are  attacked  by  water  con- 
taining carbon  dioxide  in  solution,  as  has  been  previously  explained 
(page  25) ,  with  formation  of  soluble  bicarbonates,  it  is  obvious  that 
under  the  action  of  atmospheric  water,  which  always  contains  this 
gas  to  a  greater  or  lesser  extent,  such  rock  masses  as  those  men- 
tioned must  be  continually  dissolving  and  wasting  away.  This  is 
shown  in  the  fact  that  in  those  places  where  limestone,  or  calcareous 
sandstone,  is  the  bed-rock  the  water  is  always  hard,  i.  e.,  contains 


UNDERGROUND   WATER 


163 


lime  in  solution,  as  proved  by  the  deposit  formed  in  tea-kettles.  This 
work  is  most  strikingly  ^illustrated  in  the  formation  of  sink-holes  and 
caverns,  which  result  from  underground  drainage. 

Sink-holes  and  Caverns.  —  In  regions  where  limestones  form  the 
bed-rock  the  surface  waters  working  down  through  joints  and  fis- 


Fig.  133.  —  Sink-hole  in  limestone  beds;   near  Cambria,  Wyo.     N.  H.  Darton,  U.  S. 

Geol.  Surv. 


Fig.  134.  —  Diagram  illustrating  the  formation  of  caverns  and  sink-holes  in  lime- 
stones. A  A,  clay  beds;  B  B,  limestones.  The  arch  is  the  remnant  of  the- roof  of 
a  former  cave,  forming  a  natural  bridge.  D  D,  sinks,  leading  to  domes  below. 
Modified  from  Shaler. 

sures  may  enlarge  these  by  solution.  Coming  to  an  insoluble  layer, 
such  as  one  of  clay  or  shale,  they  are  stopped  in  their  descent  and 
spread  laterally,  finding  their  way  through  the  rock  fissures  along 
the  natural  drainage  slope.  These  fissures  are  also  enlarged  by  so- 
lution until  they  become  distinct  water  channels.  As  the  latter  en- 
large they  form  caverns,  while  the  holes  or  pipes,  leading  down  to 
them  from  the  surface  above,  are  termed  sink-holes.  The  process 
is  illustrated  in  Figs.  133  and  134. 


164 


TEXT-BOOK   OF   GEOLOGY 


The  cavern  domes  hollowed  out  in  the  rock  are  sometimes  100  feet  high,  or 
more,  and  several  hundred  feet  broad.  They  are  connected  by  intricate  pass- 
ages. The  floor  on  the  insoluble  stratum  may  be  quite  level  for  long  dis- 
tances. Breaking  through  this  layer,  the  waters  excavate  new  passages 
and  chambers  at  a  lower  level,  as  illustrated  in  the  diagram,  Fig.  134,  until 
there  may  be  several  sets  of  such  rooms  and  galleries,  one  above  the  other. 

The  limestone  regions  of  the  middle  West  and  South  are  noted  for  their 
caverns,  some  of  the  best  known  being  Mammoth  Cave  in  Kentucky,  10  miles 
long  or  more,  with  30  miles  of  winding  passages ;  Wyandotte  Cave  in  Indiana, 
Luray  Cavern  in  Virginia,  and  many  others.  In  some  places  the  rocks  are 
almost  honeycombed  with  them. 


Fig.  135.  —  Silver  Spring,  Florida.     G.  I.  Adams,  U.  S.  Geol.  Surv. 

It  may  happen  in  such  regions  that  almost  the  entire  drainage  passes  under- 
ground. Large  rivers  disappear  from  sight  and,  after  a  devious  journey  below, 
may  come  to  the  surface  again  in  a  different  drainage  area.  In  thus  issuing 
they  may  give  rise  to  huge  springs,  thus  Silver  Spring  in  Florida  has  an  over- 
flow so  large  that  the  resulting  stream  is  navigable  for  small  steamers,  see 
Fig.  135.  In  other  cases  they  may  boil  up  as  great  springs  in  the  sea,  not  far 
from  land. 

In  addition  to  the  caverns  described  above,  limestones,  where  they  are  ex- 
posed to  the  weather,  commonly  show  pitted,  hollowed,  or  cavernous  sur- 
faces, owing  to  the  solvent  activity  of  water.  The  same  is  true  of  calcareous 
sandstones,  those  with  a  cement  of  carbonate  of  lime;  the  latter  dissolving, 
the  sand  grains  fall  apart  and  are  washed  or  blown  away.  Many  strange  and 
often  weirdly  shaped  masses  of  rock  are  left  as  remnants  through  this  com- 
bined chemical  and  mechanical  erosion,  see  Fig.  132. 

Deposition  and  Cementation.  —  From  what  has  been  previously 
stated  it  is  clear  that  there  is  an  upper  belt  in  the  earth's  crust 
where  mechanical  and  chemical  changes  and  destruction  are  going 


UNDERGROUND   WATER  165 

on.  It  is  known  as  the  zone  of  weathering,  and  extends  downward 
to  the  level  of  ground-water.  From  this  zone,  material  is  being  con- 
stantly leached  out  and  carried  downward  in  solution  into  the 
ground-water.  This  matter  is  either  carried  away  by  the  drainage, 
or  it  may  be  deposited  in  pores,  fissures,  and  cavities  in  the  rocks. 
The  lower  limit  to  which  this  can  extend  is  uncertain,  and  appears 
to  depend  on  several  conditions,  and  it  probably  varies  in  different 
places.  Thus  the  intervention  of  impervious  rock-layers,  or  the 
charging  of  the  rock-pores  with  gas  under  pressure,  would  hinder, 
or  perhaps  prevent,  further  downward  movement  in  a  given  area.  In 
this  belt,  whatever  its  thickness  may  be,  the  rocks  are  being  solidi- 
fied and  cemented  by  the  silica  (quartz),  carbonate  of  lime  (calcite), 


Fig.  136.  —  One  of  the  terrace  formations  of  the  Mammoth  Hotsprings,  Yellowstone 

Park,  Wyo. 

and  other  substances  deposited  in  them,  and  it  is,  therefore,  known 
as  the  zone  of  cementation.  A  proper  appreciation  of  this  process 
and  its  results  is  of  great  importance,  for  by  it  we  are  able  to  under- 
stand the  significance  of  certain  geological  features  which  will  be 
considered  later. 

Deposits  of  Carbonate  of  Lime  by  Springs.  —  The  material  in 
solution,  which  is  not  deposited  in  the  rocks,  is  carried  away  by  the 
drainage.  Sometimes  it  happens  that  on  its  way  to  the  sea  it  again 
comes  to  the  surface,  and  is  temporarily  deposited.  This  is  best 
illustrated  in  the  case  of  springs  which  deposit  carbonate  of  lime. 


166  TEXT-BOOK   OF   GEOLOGY 

Many  springs,  and  especially  deep  or  fissure  ones,  contain  carbon 
dioxide  gas,  often  in  quantity  and  under  considerable  pressure,  and 
thus  when  the  water  passes  through  beds  of  limestone  on  its  upward 
way  large  quantities  of  lime  carbonate  are  taken  into  solution,  the 
amount  depending  on  that  of  the  gas  under  pressure.  On  arriving 
at  the  surface,  partly  through  evaporation  and,  partly  by  loss  of  gas 
through  the  relief  of  pressure,  the  lime  carbonate  is  deposited,  and  in 
this  way  mounds  and  formations  may  be  built  up,  which  often  dis- 
play striking  features,  and  are  of  great  beauty.  They  are  illustrated 
in  the  basins  and  terraces  of  the  Mammoth  Hotsprings  in  the  Yel- 
lowstone Park,  Fig.  136. 

In  some  springs,  especially  deep  ones,  the  issuing  water  may  be  warm,  or 
even  hot.  This  is  apt  to  be  the  case  when  they  occur  in  regions  of  active  or 
recently  extinct  volcanic  activity,  like  that  in  which  the  Mammoth  Hotsprings 
are  situated.  In  warm  waters  the  deposit  of  lime  carbonate  may  be  much  in- 
creased by  the  action  of  low  forms  of  vegetable  life,  algae,  living  in  them 
which  secrete  this  substance  from  the  water.  It  is  probable  that  the  warmth 
and  chemical  activity  of  the  waters  of  some  springs,  particularly  hot  ones  in 
volcanic  regions,  are  greatly  increased  by  gases  and  vapors  coming  from 
molten  or  heated  rock  masses  lying  in  the  depths  below.  As  water  is  be- 
lieved to  be  the  most  considerable  of  these,  the  volume  of  water  discharged 
may  be  increased  by  this  agency. 

In  many  cases  the  deposit  takes  place  so  rapidly  that  articles  suspended  in 
the  spring  become  covered  in  a  few  days  with  a  coating  of  carbonate  of  lime. 

Other  examples  of  such  springs  are  found  in  Virginia,  Colorado,  Banff  in 
Alberta,  Karlsbad  in  Bohemia,  Tuscany,  and  in  many  other  places.  In  ad- 
dition to  carbonate  of  lime,  spring  waters  often  contain  other  mineral  sub- 
stances in  solution,  sometimes  entirely  replacing  it,  and  such  mineral  springs 
are  often  used  medicinally,  as  at  Saratoga  and  other  health  resorts. 

Deposits  in  Caves.  —  The  same  process  which  forms  caverns 
also  tends  to  fill  them  up.  For,  after  they  have  been  made  by 
underground  drainage  in  the  manner  described  above,  the  surface 
waters  seeping  down  through  the  rock-beds  which  form  their  roofs 
dissolve  more  carbonate  of  lime,  and  deposit  it  in  them,  producing 
stalactites  and  stalagmites,  columns,  pillars,  etc.  The  manner  of 
their  formation  is  as  follows:  A  drop  of  water,  charged  with  lime, 
leaking  through  to  the  roof  hangs  there  for  a  time.  While  resting 
it  evaporates  somewhat,  and  also  loses  some  carbon  dioxide,  and, 
consequently,  deposits  some  lime  carbonate.  Finally,  as  it  gathers 
volume,  it  drops,  and  falling  on  the  floor  below* it  repeats  the  process, 
leaving  another  deposit.  Thus  there  gradually  grow  downward  from 
the  roof  long  pendant  incrustations,  like  icicles,  which  are  called 
stalactites,  while  the  rising  deposits  on  the  floor  are  known  as 
stalagmites.  Finally,  these  may  increase  so  that  they  unite  and 


UNDERGROUND   WATER 


167 


produce  columns.    They  are  especially  liable  to  form  along  lines  of 
fissure  in  the  roof.    See  Fig.  137. 

In  this  way  formations  of  great  beauty,  and  often  exhibiting  many  strange 
and  curious  forms,  have  been  produced.  In  past  times  caves  have  served  as 
refuges  for  primitive  men,  who  inhabited  them,  or  as  dens  for  wild  animals. 
Through  this  the  bones  of  men  and  animals,  stone  implements,  and  other 
objects  have  accumulated  in  them  and  been  sealed  up,  like  fossils,  in  the  de- 
posits of  carbonate  of  lime  on  their  floors,  to  reveal  to  us,  when  broken  open 
and  explored,  much  concerning  the  life  and  degree  of  culture  existing  in  pre- 
historic times. 


Fig.  137.  —  Stalactites,  passing  below  into  stalagmites,  along  a  roof-crack.     Marengo 
Cave,  Indiana.     G.  P.  Merrill,  U.  S.  Nat.  Mus. 

Nature  of  Lime  Deposits ;  Travertine,  Tufa.  —  The  character  of 
the  material  formed  when  carbonate  of  lime  is  deposited  from  solu- 
tion depends  on  circumstances,  and  especially  on  the  rate  of  de- 
position. When  produced  by  slow  evaporation,  as  in  the  stalactites 
in  caves,  it  is  a  hard,  compact,  more  or  less  crystalline  substance. 
A  general  name  for  deposits  of  carbonate  of  lime  from  solution  is 
travertine,  from  the  old  Roman  name  of  a  town  (Tivoli)  in  Italy 
where  an  extensive  formation  of  the  substance  exists.  The  so-called 


Mexican  "onyx" 


or  "onyx  marble"  is  a  travertine  with  banded 


168 


TEXT-BOOK   OF   GEOLOGY 


structure  brought  out  by  varied  tinting  from  metallic  oxides.  But 
when  formed  rapidly  from  springs,  the  travertine  may  be  porous  or 
loose,  or  coating  vegetation  it  may  be  spongy  or  mosslike,  and  such 
less  compact  varieties  are  commonly  called  calcareous  tufa,  or  some- 
times calcareous  sinter.  Great  deposits  of  this  are  also  found  around 
the  shores  of  dried-up  alkaline  lakes,  such  as  Pyramid  Lake  in  Ne- 
vada, encrusting  the  rocks  of  the  enclosing  basin,  as  mentioned  on 
page  86. 

It  should  be  clearly  borne  in  mind  that  these  deposits  are  not 
original  formations  of  carbonate  of  lime,  in  the  sense  in  which  we 


Fig.  138.  —  Alkali  flat,  Malheur  Lake,  Oregon.     I.  C.  Russell,  U.  S.  Geol.  Surv. 

might  think  of  that  word  in  connection  with  limestone;  they  repre- 
sent, certainly  for  much  the  greatest  part,  previously  existent  car- 
bonate of  lime,  such  as  limestone,  chalk,  etc.,  which  has  gone  into 
solution,  been  transferred  to  another  place,  and  deposited.  They 
merely  exhibit  a  temporary  stoppage  of  the  material  on  its  way  to 
the  sea,  for  it  is  the  fate  of  all  deposits  of  carbonates,  exposed  to 
atmospheric  agencies,  to  be  dissolved  and  taken  into  the  ocean. 
Some  have  even  had  the  view  that  thick  formations  of  limestone, 
covering  wide  areas,  have  thus  dwindled  and  disappeared,  but  this 
idea  may  be  carried  too  far.  What  happens  to  the  carbonates  in 
the  sea,  and  how  the  limestones,  which  furnish  the  secondary  de- 
posits of  travertine  and  tufa,  were  made  we  shall  see  in  a  later  place. 
Other  Deposits  by  Springs;  Iron  Oxides,  Silica,  etc.  —  Sub- 
stances other  than  travertine  may  be  deposited  when  underground 
waters  issue  at  the  surface.  One  of  these  is  the  hydrated  oxide  of 
iron,  or,  under  certain  circumstances,  iron  carbonate.  This  is  a 
matter  of  importance  because,  as  is  commonly  supposed,  extensive 


UNDERGROUND   WATER  169 

beds  of  valuable  ore  have  been  thus  formed.  Also  silica,  sulphur, 
and  gypsum  may  be  deposited,  but  since  agencies  other  than  those 
which  have  thus  far  been  described,  are  also,  as  a  general  thing, 
concerned  in  the  process  it  is  better  to  wait  until  these  latter  have 
been  considered  before  discussing  them. 

Alkali  Deposits.  —  In  humid  regions  the  soluble  substances  that 
are  formed  in  the  decay  of  the  rocks  are  quickly  washed  out  of  the 
soil,  and  passing  into  the  drainage  are  carried  into  the  sea.  In  arid 
and  desert  regions  where  the  rainfall  is  scanty  there  may  not  be 
sufficient  water  to  perform  this  function.  The  salts  remain  in  the 
soil,  at  times  of  rainfall  they  go  into  solution,  and  in  the  subsequent 
times  of  dryness,  when  the  water  draws  to  the  surface,  on  its  evapo- 
ration they  are  left,  forming  the  white  incrustation  on  the  soil  known 
as  alkali,  a  common  feature  in  many  parts  of  our  western  regions, 
see  Fig.  138. 

The  common  salts  in  the  so-called  alkali  are  sodium  sulphate,  sodium  chlo- 
ride, and  sodium  carbonate,  Na2SO4,  NaCI,  and  Na2CO3 ;  it  is  to  the  alkaline 
reaction  and  taste  of  the  latter  that  the  name  is  due.  Magnesium  sulphate, 
MgS04,  and  sulphate  of  lime,  or  gypsum,  CaS04  .  2H2O,  are  often  present. 
These  salts  are  not  always  formed  by  rock  decay;  they  may  have  been  origi- 
nally present  in  the  rocks,  if  these  are  composed  of  beds  of  sediments  laid 
down  in  the  sea.  Their  concentration  in  such  arid  regions,  with  inland  drain- 
ages, gives  rise  to  salt  and  alkaline  lakes.  See  page  86.  The  irrigation  of 
alkali  lands,  especially  if  the  water  is  too  freely  or  carelessly  used,  may  bring 
the  salts  to  the  surface  in  such  quantities  as  to  injure,  or  even  ruin  them  for 
agriculture. 

Mechanical  Work  of  Water  Underground ;  Landslides.  —  As  a 
mechanical  agent  underground  water  must  play  a  small  geological 
role.  It  is  conceivable  that  streams  running  in  subterranean  chan- 
nels may  at  times  both  erode  and  transport,  but  the  circumstances 
which  would  permit  this  must  be  exceptional.  A  more  important 
function  is  its  aid  in  causing  landslides,  both  in  helping  to  overcome 
the  friction  of  masses  of  rock,  earth,  and  debris  lying  on  steep  slopes, 
and  in  adding  weight  to  such  masses.  In  producing  such  results  it 
is  often  powerfully  aided  by  the  action  of  frost,  as  mentioned  on 
page  21.  The  masses  of  earth  and  rock  when  saturated  with  water 
act  like  a  semi-fluid  substance  and,  started  from  their  insecure 
foundations  at  times  of  extraordinarily  heavy  rainfall  or  by  earth- 
quake shock,  rush  down  into  the  valleys  below,  often  causing  great 
damage  and  considerable  changes  of  topography.  In  high  moun- 
tainous regions  such  landslides,  due  to  these  causes  and  shattered 
condition  of  the  rock  masses,  may  precipitate  huge  trains  of  broken 


170  TEXT-BOOK   OF   GEOLOGY 

rock,  or  talus  heapings,  for  long  distances  downward,  giving  rise  to 
rock  streams.  The  onward  motion  of  trains  of  talus,  or  "rock 
glaciers,"  as  they  have  been  sometimes  called,  due  to  freezing  and 
thawing,  and  to  gravitational  creep,  has  been  already  stated  on 
page  118. 


CHAPTER  VII 
ORGANIC  LIFE  AND  ITS  GEOLOGICAL  WORK 

Over  the  greater  part  of  the  land  surfaces  and  in  the  sea,  life, 
both  animal  and  vegetable  in  varied  forms,  is  present  and,  quick- 
ened by  the  energy  imparted  by  heat  and  light  from  the  sun,  is 
causing  movement  of  material  on  the  earth's  surface  and  transfor- 
mations of  matter  by  chemical  changes.  Compared  with  the  vast 
bulk  of  the  globe  such  actions,  and  their  results,  appear  relatively 
superficial  and  small;  from  the  human  standpoint,  however,  they 
are  not  only  great,  but  of  far-reaching  importance  and  worthy  of 
careful  consideration. 

Organisms  work  in  various  ways:  in  some  cases  they  tend  to 
break  down  existing  structures  and  their  action  is  thus  destructive; 
in  others  they  build  new  ones  and  their  work  is  therefore  construc- 
tive. Sometimes  they  preserve  existent  structures  from  the  destroy- 
ing action  of  other  agencies,  and  thus  are  protective.  The  most  im- 
portant protective  effect  is  the  influence  of  vegetation  in  restraining 
the  erosion  of  the  soil,  a  matter  which  has  already  been  sufficiently 
treated  (page  3  -)  under  erosion. 

Destructive  Work  of  Organisms 

Destructive  Work  of  Plant  Life.  —  The  most  important  geologi- 
cal process  which  plant  life  carries  on  in  growing  is  to  decompose 
the  carbon  dioxide  gas  in  the  atmosphere,  storing  up  the  carbon  and 
returning  to  it  the  oxygen.  This  produces  several  important  effects 
which  will  be  considered  in  their  proper  places.  On  their  death  and 
decay  the  carbon  of  the  vegetable  tissues  of  the  plants  may  be 
largely,  or  even  wholly,  reoxidized  to  carbon  dioxide,  and  this  being 
taken  into  solution  by  the  descending  surface  waters  forms  carbonic 
acid,  which,  as  we*  have  already  seen  in  several  places,  is  a  solvent 
of  rock  material. 

The  carbonaceous  residue  from  the  decay  of  vegetation  exist- 
ing in  the  soil,  which  it  colors  dark,  or  black,  is  known  as  humus.  In 
the  production  of  humus,  not  only  carbonic  acid,  but  other  com- 
pounds are  formed,  some  of  which  are  organic  acids  called  humic, 
ulmic,  etc.  These  also  attack  the  rocks  and  help  to  convert  them 

171 


172  TEXT-BOOK   OF   GEOLOGY 

into  soil.  Even  the  roots  of  growing  plants  excrete  carbon  dioxide 
and  contain  organic  acids,  such  as  citric  acid,  which  exert  a  solvent 
influence  on  the  minerals  composing  the  rocks.  Thus  in  its  life, 
death,  and  decay,  vegetation  is  exerting  a  constant  chemical  effect 
upon  the  soil  and  rocks,  changing  the  existing  substances  into  new 
ones,  many  of  which  are  soluble  and  carried  away  by  the  circulating 
waters.  This  is  most  strikingly  seen  in  its  effect  upon  the  oxides  of 
iron  which  color  the  soils  red  or  yellow.  They  consist  of  ferric  oxide, 
Fe203,  usually  more  or  less  hydrated,  as  in  limonite,  2  Fe2O3.H20, 
which  makes  yellow  ocher  when  mixed  with  clay.  Decay  of  organic 
substance  is  a  process  of  oxidation ;  mostly  the  oxygen  is  taken  from 
the  air,  but  if  the  organic  material  is  in  contact  with  ferric  oxide  it 
will  also  take  oxygen  from  it,  reducing  it  to  ferrous  oxide,  FeO.  The 
ferrous  oxide,  however,  as  it  forms,  unites  with  the  carbon  dioxide, 
also  being  produced,  and  makes  ferrous  carbonate,  FeC03.  The 
process  may  be  reduced  to  simple  chemical  equations  by  considering 
the  organic  substances,  which  really  consist  of  carbon,  hydrogen 
and  oxygen,  as  if  composed  of  pure  carbon,  as  follows: 

2  Fe203  +  C  =  4  FeO  +  C02,  and  FeO  +  C02  =  FeC03. 

The  ferrous  carbonate,  like  calcium  carbonate,  is  soluble  in  water 
containing  carbon  dioxide,  and  as  this  is  always  present  to  a 
greater  or  lesser  extent,  the  iron  compound  is  taken  into  solution, 
leached  out  and  carried  away.  Thus  while  ferric  oxide,  so  common 
a  coloring  material  and  cement  in  rocks  and  soils,  is  insoluble  in 
meteoric  waters,  by  the  aid  of  organic  matter  it  is  converted  into 
the  soluble  ferrous  carbonate,  dissolved  and  removed.  What  be- 
comes of  it  we  shall  consider  later. 

Certain  features  regarding  the  coloring  of  rocks  and  soils  are  explained  by 
this  process.  Thus  the  soil  lying  below  a  covering  of  vegetable  mold 
(humus)  is  usually  decolorized  and,  therefore,  of  light  hue,  or  white,  because 
the  solutions  of  organic  matter  leaching  downward  from  above  have  changed 
the  iron  oxide  and  removed  it.  This  may  be  often  observed  on  the  sides  of 
banks,  railway  cuttings,  and  excavations. 

On  the  other  hand,  a  clay  which  contains  much  organic  matter  is  dark  in 
color,  dark-blue,  dark-gray,  or  greenish  to  black.  In  such  clay,  iron,  if  present, 
is  in  ferrous  compounds  on  account  of  the  reducing  action  of  the  organic  sub- 
stances and,  as  ferrous  compounds  have  little  or  no  coloring  effect,  the  color- 
ation is  due  to  the  dark  carbonaceous  material.  When  these  clays  are  fired, 
however,  the  organic  matter  is  burned  out,  the  iron  oxidized  to  the  ferric 
condition,  and  red  brick  formed. 

In  desert  or  very  arid  regions  the  cliffs  and  rocks  are  often  strongly  colored 
red,  or  less  often  yellow,  because  of  the  oxidation  of  the  iron  compounds  in 
them  and  the  lack  of  vegetation  in  such  places,  whose  decay  would  reduce 
the  ferric  compounds  and  decolorize  them.  Thus  vivid  color  tones  are  often 


ORGANIC   LIFE   AND   ITS   GEOLOGICAL   WORK 


173 


characteristic  of  many  arid  landscapes.  The  prevailing  color  of  sandy  deserts 
is,  however,  gray,  often  pale  yellow,  less  often  red. 

With  regard  to  the  soils  it  may  be  said,  in  general,  that  while  a  red  color  is 
characteristic  of  many  of  the  residual  soils  of  to-day  in  warm  moist  climates, 
in  ancient  deposits,  now  hardened  into  rock,  it  is  apt  to  be  associated  with 
salt  and  gypsum,  which,  as  will  be  shown  later,  are  indicative  of  arid  conditions. 

The  importance  of  the  principles  here  laid  down  will  be  seen  later,  when  the 
climates  of  past  times  in  different  places,  and  their  significance,  are  treated, 
and  the  formation  of  beds  of  iron-ore  is  discussed. 


Fig.  139.  —  Rock  split  by  a  tree  growing  from  a  seed  which  lodged  in  a  crack. 
Nevada,  Cal.     G.  K.  Gilbert,  U.  S.  Geol.  Surv. 


Sierra 


In  addition  to  the  chemical  work  of  plant  life  just  described,  vege- 
tation also  acts  in  a  mechanical  way  to  destroy  existent  structures. 
The  most  important  part  of  this  work  is  seen  in  the  splitting  and 
disintegration  of  rocks  by  the  roots  of  trees,  shrubs,  and  other 
plants.  They  insinuate  themselves  when  minute  into  crevices  and, 
expanding  as  they  grow,  they  exert  a  disruptive  force  which  even 
solid  masses  of  rock  are  unable  to  withstand.  Instances  of  this  in 
exposed  ledges  and  bowlders,  where  seeds  have  lodged  in  cracks,  and 
have  germinated  and  grown,  enlarging  the  cracks  and  disrupting  the 
rock,  are  everywhere  common.  Such  a  case  is  illustrated  in  Fig. 
139.  In  the  course  of  long  ages  the  amount  of  work  done  in  this 


174  TEXT-BOOK   OF   GEOLOGY 

way,  especially  on  rocks  in  the  soil,  must  be  very  great,  and  the  gen- 
eral action  of  weathering  facilitated  by  the  preliminary  effect  of 
roots. 

Destructive  Work  of  Animals.  —  Animals  are  much  less  de- 
structive agents  than  plants  yet  on  the  whole  they  accomplish  con- 
siderable geological  work.  It  is  done  chiefly  by  those  kinds  which 
live  and  move  about  in  the  soil,  such  as  worms,  ants,  moles,  gophers, 
etc.  By.  making  holes  and  burrows,  and  upturning  the  soil  they  ex- 
pose fresh  surfaces  to  weathering  and  erosion,  or  by  opening  it  up 
they  facilitate  the  entrance  of  the  weathering  agents  to  lower  levels. 
Thus  Darwin  states  as  the  result  of  his  investigations  that  in  Eng- 
land the  earthworms  bring  to  the  surface  10  tons  of  mold  to  the 
acre  every  year,  while  Branner  believes  that  in  many  tropical 
regions  the  ants  are  even  more  effective  in  upturning  the  soil. 

The  most  destructive  animal  in  regions  populated  by  him  is  Man, 
and  this  is  due  to  the  fact  that  over  such  wide  areas  he  has  felled 
the  forests,  and  otherwise  destroyed  the  natural  vegetal  covering  of 
the  soil,  in  order  to  cultivate  it,  thus  throwing  it  open  to  attack  by 
the  agencies  of  erosion.  This  has  already  been  discussed  under  ero- 
sion, page  33.  The  work  of  man  as  a  geological  agent  is  also  seen 
in  the  diversion  of  drainages  he  has  effected  by  wells,  canals,  dams, 
piers,  dredgings,  etc.,  although  all  of  this  is  by  no  means  destructive 
in  its  nature.  The  extermination  of  animals  and  plants,  and  the  in- 
troduction of  new  species  of  both,  in  the  settlement  of  new  countries 
by  man,  is  also  a  process  which  has  a  geological  bearing,  and  has 
been  going  on  at  an  increasing  rate  for  an  immense  period  of  time, 
since  man  first  definitely  assumed  his  position  as  master  of  living 
organisms. 

There  should  be  mentioned  here  also  the  chemical  changes 
wrought  by  the  decay  of  dead  organisms  in  the  sea,  much  of  which  is 
destructive  in  character.  Both  plants  and  animals  contribute  to 
these  changes,  which  take  place  chiefly  on  the  bottom.  The  de- 
composing organic  matter  reduces  the  sulphates  in  sea  water  to 
sulphides,  with  consequent  formation  of  sulphuretted  hydrogen, 
H2S,  which  may  precipitate  sulphides,  such  as  pyrite,  FeS2,  or  be 
oxidized  to  sulphuric  acid,  H2S04.  The  acid  may  attack  the  lime 
carbonate  in  shells,  and  convert  it  into  gypsum,  CaS04  -H2O. 
This  may  serve  as  an  example  of  chemical  changes  going  on  in  the 
sea  through  the  agencies  of  organic  life.  Carbonates,  sulphates  and 
phosphates  are  the  chief  results.  Diagenesis  is  a  general  term  for 
these  processes,  which  are  of  great  importance  in  various  ways,  as  we 
shall  see  later  when  we  shall  have  occasion  to  refer  to  them. 


ORGANIC   LIFE   AND   ITS   GEOLOGICAL   WORK  175 

Constructive  Work  of  Organisms 

Constructive  Work  of  Plants.  —  The  manner  in  which  plant  life 
acts  as  a  constructive  geological  agent  is  best  seen  in  the  formation 
of  peat  and  its  conversion  of  lakes  into  bogs  and  swamps.  This  has 
been  briefly  mentioned  in  the  life  history  of  lakes,  page  82,  and 
may  now  be  more  fully  considered.  We  will  commence  by  learning 
what  peat  is,  and  how  it  is  formed. 

Peat.  —  It  has  been  previously  stated  that  growing  plants  de- 
compose the  carbon  dioxide  of  the  atmosphere,  using  the  carbon  and 
largely  returning  to  it  the  oxygen.  In  addition  they  demand  water, 
which  consists  of  oxygen  and  hydrogen,  and  certain  mineral  com- 
pounds furnished  them  by  the  soil.  Their  tissues,  therefore,  consist 
chiefly  of  carbon,  hydrogen,  and  oxygen,  but  contain  mineral  sub- 
stances, and  in  some  cases  nitrogen  also  in  small  amount.  Disre- 
garding the  minor  constituents,  the  chief  substance  composing  their 
frame-work  is  cellulose,  C6H1005,  which,  for  the  sake  of  simplicity 
in  this  connection,  we  may  regard  as  forming  the  organic  matter  of 
plant  life. 

If  dried  organic  matter,  or  cellulose,  be  burned  with  free  access 
of  air,  a  complete  process  of  oxidation  takes  place,  with  formation  of 
carbon  dioxide  and  water  vapor,  as  follows: 

C6H1005  +  12  0  ==  6  C02  +  5  H20. 

If  the  heating,  or  burning,  is  conducted  without  access  of  air,  or  of 
but  a  limited  amount,  as  when  wood  is  charred  in  a  kiln,  or  with 
earth  thrown  over  it,  the  oxidation  is  incomplete,  the  hydrogen,  oxy- 
gen and  some  of  the  carbon  are  removed,  partly  as  above,  but  the 
greater  part  of  the  carbon  remains  as  charcoal.  Somewhat  similar 
processes  take  place  in  nature  when  organic  matter  decays.  Decay 
is  caused  by  the  growth  and  action  of  bacteria,  minute  organisms, 
and  is  a  process  of  oxidation.  If  it  takes  place  in  the.  open  air,  as 
when  leaves  fall  upon  the  ground,  it  may  be  in  time  complete,  and 
the  cellulose  returned  to  the  atmosphere  as  carbon  dioxide  and 
water  vapor,  as  if  it  had  been  burned. 

On  the  other  hand,  if  the  organic  matter  decays  where  the  access 
of  air  is  prevented,  as  when  plants  grow  in  water,  or  their  leaves, 
twigs,  stems,  etc.,  in  falling  pass  beneath  its  surface,  a  process  some- 
what analogous  to  the  formation  of  charcoal  takes  place.  The  oxi- 
dation is  only  partial,  some  of  the  hydrogen  being  removed  as 
water,  H20,  some  of  the  carbon  as  carbon  dioxide,  CO2,  and  some  of 
both  as  marsh  gas,  CH4.  The  resulting  product  of  partly  decayed 
organic  matter  is  much  richer  in  carbon  and  poorer  in  hydrogen  than 


176 


TEXT-BOOK    OF    GEOLOGY 


the  original  material,  and  is  known  as  peat.  Peat  is,  therefore,  the 
brown  to  black  carbonaceous  matter  formed  by  the  partial  decay  of 
vegetable  matter  under  a  protective  covering  of  water. 

The  reason  for  the  arrest  of  decay  in  this  case  appears  to  be  that  the  bac- 
teria producing  it  evolve  waste  products,  which,  if  not  removed,  are  un- 
favorable to  their  continued  growth  and  existence,  that  is,  they  are  antiseptic 
in  nature.  When  the  condition  is  reached  that  the  water  saturating  the  decay- 
ing organic  matter  is  changed  to  a  sufficiently  strong  antiseptic  solution  from 
the  presence  of  these  substances,  the  bacteria  can  no  longer  exist,  further  decay 
is  prevented,  and  the  peat  formed  is  preserved. 

Formation  of  Peat  and  Lake  Filling ;  Bogs.  —  Although  peat  is 
formed  to  some  extent  in  warm  and  even  tropical  regions,  it  is  es- 


Fig.  140.  —  A,  bed-rock  of  lake  basin;  B,  accumulating  layer  of  peat;  C,  aquatic 
vegetation,  pond-lilies,  water-weeds,  etc.;  D,  bushes  and  semi-aquatic  plants, 
mosses,  etc.;  JE,  climbing  bog.  Modified  from  Shaler. 

pecially  in  temperate  and  cold  humid  countries  that  it  is  produced. 
Some  of  the  various  circumstances  under  which  this  happens,  and 
their  results,  are  discussed  in  the  following  paragraphs.  Thus, 
where  lakes  abound,  especially  in  humid  regions,  constant  formation 
of  peat  in  shallow  water  is  going  on,  and  is  slowly  but  steadily 
filling  them  up.  In  the  water  are  growing  various  kinds  of  aquatic 
vegetation,  pond-lilies,  water-weeds,  rushes,  etc.  When  these  die 
their  leaves,  stems,  and  roots  at  the  bottom  form  a  black  mud  com- 
posed of  peat.  As  these  masses  of  vegetation,  and  the  deposits  they 
leave  behind  them,  advance  lakeward,  bushes  and  semi-aquatic 
plants,  such  as  certain  mosses,  appear  in  the  shallowing  water,  and 
close  to  the  shore,  and  add  their  quota  to  the  peat  deposits  below. 
This  is  illustrated  in  the  diagram,  Fig.  140. 

Eventually  there  comes  a  time  when  the  peat  formation  reaches 
to  the  top,  or  nearly  so,  the  basin  is  filled  with  the  soft  black  mud 
which  forms  the  final  stage  of  the  peat,  the  lake  is  obliterated  and 
a  bog  formed  in  its  place.  See  Fig.  141. 

This  process  is  especially  important  in  small  lakes  and  ponds,  and  in  the 
shallow  bays  and  lagoons  formed  by  bars  or  barriers  in  large  lakes,  where  the 
depth  is  not  too  great  for  plant  life  to  gain  a  foothold.  In  the  larger  and 
deeper  lakes  it  may  be  at  first  a  relatively  unimportant  factor  in  filling,  com- 


ORGANIC   LIFE   AND   ITS   GEOLOGICAL    WORK 


177 


pared  with  the  deposits  produced  by  incoming  sediments,  but  when  the  stage 
is  reached  where  vegetation  becomes  abundant  this  may  be  reversed. 

In  northern  regions  the  plants  most  efficient  in  forming  peat  are  species 
of  mosses,  especially  sphagnum  (bog-moss) ,  sedges,  and  certain  flowering  plants 
which  grow  rapidly,  producing  a  spongy,  cushion-like  layer  saturated  with 
water.  While  growing  above,  the  stems  die  below,  making  the  peat.  As  in- 
dicated in  Fig.  140  the  plants  tend  to  grow  in  definite  zones;  first  the  floating 
lilies,  next  shoreward  the  sedges,  rushes,  etc.,  followed  by  mosses,  land 


Fig.  141.  —  Lake  filling;  final  stage  where  it  is  turned  into  a  bog  by  accumu- 
lated peat.     Near  Hammond,  La. 

bushes,  and,  finally,  very  often  swamp-loving  trees,  larches  and  spruces,  near 
the  edge.  Where  suitable  conditions  exist,  especially  in  small  lakes,  the  vege- 
tation pushing  outward  from  the  shore  may  form  a  floating  mat.  Eventually 
when  the  lake  is  filled  by  the  deposited  peat  the  bog-moss  and  bushes  form  a 
cover  concealing  the  black  and  treacherous  quagmire  below.  Especially  in 
the  many  small,  shallow  lakes  of  glacial  origin  in  northern  countries  is  this 
filling  going  on.  Over  wide  regions,  as  in  Newfoundland,  Labrador,  etc.,  not 
only  the  surface  of  filled  lakes,  or  bogs,  but  all  shallow  depressions  and  in 
some  places  the  level  ground,  hill-slopes  and  hill-tops,  even  isolated 
rocks,  are  covered  with  this  saturated  layer,  giving  a  bog-like  aspect  to  the 
entire  country.  In  sub-arctic  regions,  as  in  Alaska  and  Siberia,  the  country 
covered  by  this  wet,  mossy  mantle  of  bog,  which  may  be  even  continually 
frozen  a  small  depth  below,  is  known  as  tundra. 

Southern  Swamps.  —  In  temperate  to  tropical  regions  the  mossy 
bogs  of  the  north  are  replaced  by  swamps  filled  with  trees,  bushes, 
canes,  vines,  etc.,  whose  decay  forms  the  peat.  Such  are  the  swamps 
along  the  lower  Mississippi  and  its  tributaries,  the  Great  Dismal 


178 


TEXT-BOOK   OF   GEOLOGY 


Swamp  in  Virginia  and  North  Carolina,  and  the  marshes  and 
swamps  of  Florida.  Dismal  Swamp  covers  an  area  30  miles  long 
by  10  broad,  and  appears  to  have  been  caused  by  the  obstruction  to 
drainage  produced  by  accumulations  of  dense  vegetation  on  a  plain 
lying  near  sea-level.  The  trees  covering  it,  of  which  the  cypress  is  the 
most  characteristic  of  this  and  other  southern  swamps,  maintain 
themselves  in  the  soft  peat  mud  by  platforms  of  wide  spreading 


Fig.  142.  —  Dismal  Swamp,  Va.  The  projections  from  the  cypress  roots  serve  to 
give  them  air;  they  extend  downward  into  the  mud  and  help  to  anchor  the  tree  in 
the  semi-liquid  mass  of  the  bog.  I.  C.  Russell,  U.  S.  Geol.  Surv. 

roots.  In  the  swamp  is  Lake  Drummond,  six  miles  in  diameter,  but 
very  shallow,  its  banks  and  bottom  composed  of  pure  peat.  A  view  in 
this  swamp  is  seen  in  Fig.  142.  In  tropical  regions,  as  in  the  basins 
of  the  Amazon  and  Nile  rivers,  vast  swamps  and  marshes  occur, 
formed  by  the  obstruction  to  drainage  caused  by  the  rapid  growth 
and  accumulation  of  vegetation  on  an  enormous  scale,  especially 
of  aquatic  kinds,  such  as  rushes,  canes,  etc.  These  also  give  rise  to 
peat  deposits. 

Marine  Marshes.  —  In  bays  and  harbors  along  sea-coasts  and 
on  the  deltas  of  large  rivers  vegetation  plays  a  prominent  part  in 
helping  to  turn  shallow-water  areas  into  marine  marshes.  For  when 
the  depth  of  water  is  sufficiently  small ?  or  becomes  so  through  de- 


ORGANIC   LIFE   AND    ITS   GEOLOGICAL    WORK  179 

posit  of  sediment,  marine  vegetation,  partly  growing  completely 
submerged,  or  aquatic,  like  eel-grass,  partly  semi- aquatic,  like  cer- 
tain grasses  and  rushes,  takes  root  and  flourishes.  At  high  tide  this 
band  of  vegetation  may  be  well  covered  with  water,  but,  as  the 
tide  recedes  and  its  current  slackens,  sediment  and  floating  matter 
borne  by  it  are  entangled  among  the  stems  in  the  fields  of  grass,  and 
sink  to  the  bottom.  The  stems,  leaves  and  roots  of  the  grasses, 
along  with  seaweeds,  on  decaying  make  peaty  material.  The  min- 
gled deposit  of  sediment  and  organic  matter  thus  rises  until  it 
reaches  high-tide  level,  new  kinds  of  fresh-water  plants  coming  in 
to  replace  the  plants  first  mentioned,  which  move  seaward  as  the 
water  shallows,  and  thus  marine  marshes  are  formed,  often  overlaid 
by  fresh-water  plants.  The  process  is  illustrated  in  Fig.  143,  and 
is  often  the  final  stage  of  filling  of  the  sounds  and  lagoons  formed 
by  wave  action,  page  108. 


Zone  of  Grass  High  Tide  Level 


Fig.  143.  —  Illustrating  the  formation  of  a  marine  marsh.  A,  sedimentary  deposits; 
B,  peaty  mud  deposit  formed  by  action  of  vegetation.  Modified  from  G.  P. 
Merrill. 

In  the  low  marshy  regions  about  the  deltas  of  great  rivers,  such  as  the  Mis- 
sissippi, which  are  sometimes  inundated  by  the  sea  and  sometimes  covered  by 
fresh  water  from  the  river  in  times  of  flood,  similar  processes  prevail,  although 
over  wide  stretches  pure  peat  may  be  the  only  deposit  laid  down,  since  the 
vegetation  may  be  so  dense  as  to  cause  the  water  to  quickly  deposit  all  its 
sediment  before  reaching  the  interior  of  the  swamp  or  marsh. 

On  the  shores  of  warm  seas,  as  on  the  coast  of  Florida,  mangroves,  which 
are  small,  many-rooted  trees  growing  only  in  sea-water,  perform  a  somewhat 
analogous  function  in  making  marshes.  Their  maze  of  roots  entangle  sedi- 
ment and  other  matter,  and  help  to  form  a  barrier  to  the  escape  of  water 
from  the  land.  By  this  means  shallow  stretches  of  sea-bottom  have  been 
changed  into  swamps  and  marshes,  as  in  parts  of  the  Everglades. 

Properties  and  Uses  of  Peat.  —  Peat  varies  from  a  brown, 
spongy,  fibrous,  or  matted  mass,  resembling  tobacco  when  least 
altered,  to  a  fine,  black,  granular  mud,  when  most  changed.  The 
latter,  when  dried  and  compressed,  much  resembles  lignite,  or  brown 
coal.  Peat,  when  cut  and  dried  in  the  form  of  turfs,  is  much  used 
in  many  countries,  especially  in  Europe,  as  a  cheap  fuel;  in  North 
America,  owing  to  the  abundance  of  wood  and  coal,  it  has,  up  to  the 


180  TEXT-BOOK   OF    GEOLOGY 

present  time,  received  little  attention.  The  amount  of  it  in  the 
United  States  in  the  various  bog  and  swamp  areas  is,  however, 
enormous,  being  estimated  by  the  Geological  Survey  at  12  billion 
tons  of  air-dried  fuel ;  with  the  increasing  scarcity  of  wood  and  up- 
ward tendency  in  the  price  of  coal,  and  the  discovery  of  the  value 
of  peat  as  a  source  of  power  in  the  gas-producer  engine,  it  will 
probably  have  a  growing  use  in  the  future. 

The  antiseptic  quality  of  peat  bogs  has  been  already  mentioned;  this  is 
strikingly  shown  in  the  preservation  of  the  bodies  of  men  and  animals  which 
became  entombed  in  them  many  hundreds,  or  even  thousands  of  years  ago. 
Trunks  of  trees  and  their  stumps  have  also  been  preserved,  and  in  some 
places  cedar  logs  thus  buried  have  been  extracted  and  used  for  the  valuable 
timber  they  afford. 

Relation  of  Peat  to  Coal.  —  The  principles  which  have  been  laid 
down  concerning  the  origin  and  formation  of  peat  are  of  the  greatest 
importance  regarding  a  correct  understanding  of  the  origin  of  coal, 
and  of  the  conditions  under  which  it  was  formed.  Peat  is  the  first 
stage  in  the  transformation  of  vegetable  matter  into  coal.  Further 
stages,  and  the  kinds  of  coal,  will  be  considered  in  a  later  place.  It 
is  important  to  observe,  however,  that  in  certain  places  where  sub- 
sidence of  the  earth's  crust  and  deposit  of  sediments  are  going  on, 
as  in  the  deltas  of  rivers  such  as  the  Mississippi  and  the  Ganges, 
borings  show  that  layers  of  peat,  often  of  considerable  thickness,  are 
found  alternating  with  beds  of  sands  and  clays,  just  as  layers  of 
coal  are  found  between  beds  of  shale  and  sandstone. 

Reclamation  of  Swamp-lands.  —  It  is  estimated  that  over 
100,000  square  miles  of  the  United  States  consist  of  swamp,  bog, 
or  inundated  land,  which  in  its  present  condition,  although  valu- 
able in  places  for  the  timber  it  contains,  is  useless  for  agriculture. 
By  the  use  of  suitably  placed  canals  and  ditches,  a  very  large,  per- 
haps the  greater,  part  of  this  land  can  be  drained  and  rendered 
available  for  cultivation.  It  generally  possesses  a  very  fertile  soil. 
Some  work  has  been  done  towards  reclaiming  these  swamp  and 
marsh  lands,  as  in  Florida,  California,  and  in  the  Dismal  Swamp, 
with  results  like  those  shown  in  Fig.  144.  With  the  closer  settle- 
ment of  the  country  and  consequent  greater  demand  for  land,  and 
with  the  initiation  of  reclamation  projects  in  these  inundated  areas 
by  the  National  Government,  we  may  expect  to  see  in  the  future  a 
constantly  increasing  use  of  swamp  lands. 

Diatom  Deposits.  —  It  has  been  already  mentioned  (page  115)  that  sili- 
ceous deposits,  which  occur  over  vast  stretches  of  the  sea  floor,  are  composed  of 


ORGANIC    LIFE   AND    ITS    GEOLOGICAL   WORK  181 

the  shells  of  diatoms,  extremely  minute  uni-celled  vegetable  organisms.  These 
also  live  in  lakes  and  marshes,  and  even  in  warm  springs  and  pools,  as  in  the 
Yellowstone  Park,  and  living  and  dying  in  almost  unimaginable  numbers  their 
shells  form  deposits,  often  of  considerable  thickness.  This  white,  porous, 
chalk-like  deposit  of  silica,  SiO2,  is  known  as  diatomaceous  earth,  or  tripolite, 
and  beds  of  it  several  hundred  feet  thick  have  been  found  in  places.  It  is 
used  for  several  purposes,  such  as  polishing  powder,  in  making  dynamite,  etc. 

Iron-ore  Deposits.  —  In  a  previous  section  (page  172)  we  have 
shown  how,  by  the  influence  of  organic  matter,  iron  in  the  rocks  and 
soils  is  reduced  from  ferric  to  ferrous  oxide,  and  taken  into  solution. 
It  now  becomes  pertinent  to  inquire  what  becomes  of  the  iron.  When 


Fig.  144.  —  Reclaimed  land  from  the  Dismal  Swamp,  Va.     I.  C.  Russell, 
U.  S.  Geol.  Surv. 

thus  brought  into  solution  it  is  leached  out,  and  in  standing  bodies 
of  shallow  water,  such  as  swamps,  lagoons,  or  estuaries  with  small 
outlets  to  the  sea,  it  may  be  concentrated  and  give  rise  to  consider- 
able deposits.  Many  of  the  beds  of  limonite  iron-ore,  extending  from 
Vermont  and  New  York  southward  to  Alabama,  are  examples  of 
these.  Under  some  conditions  these  beds  may  be  of  ferrous  car- 
bonate (siderite,  FeC03)  directly,  but  usually  the  solution  of  the 
carbonate  is  reoxidized,  carbon  dioxide  escapes,  and  the  iron  is  pre- 
cipitated as  ferric  hydroxide,  limonite,  as  follows: 

4  FeC03  +  02  +  3  H20  =  2  Fe203 . 3  H20  +  4  C02. 

The  most  interesting  feature  of  the  process  is  that  the  oxidation  from  the 
ferrous  to  the  ferric  condition  is  largely  performed  by  certain  exceedingly 
minute  vegetable  organisms  living  in  the  water,  which  are  known  as  the  iron 
bacteria.  These  secrete  the  iron  from  solution,  and  change  it  in  their  cells 
from  the  ferrous  to  the  ferric  condition,  thus  rendering  it  insoluble.  Although 
so  excessively  minute,  yet  occurring  in  such  enormous  numbers,  they  may  give 
rise  to  large  deposits. 


182  TEXT-BOOK   OF   GEOLOGY 

The  ferric  hydroxide  thus  precipitated  may  accumulate  on  the 
bottom  as  bog  iron-ore,  or  limonite,  or,  as  in  swamps,  it  may  again 
come  in  contact  with  decaying  organic  matter  and  be  changed  back 
into  ferrous  carbonate.  Such  beds  of  ore  may  be  quite  pure,  or, 
mingled  with  clay  or  sand,  they  may  form  deposits  of  impure  limo- 
nite, clay-ironstone  (FeC03),  etc.  This  may  also  explain  the  fre- 
quent occurrence  of  beds  of  iron-ore  and  of  coal  (ancient  peat  bed) 
in  the  same  series  of  stratified  rocks,  and  why,  in  this  case,  the  ore 
is  so  often  ferrous  carbonate.  Some  are  inclined  to  believe  that  all 
beds  of  iron-ore  found  in  the  stratified  rocks  are  due  to  these  pro- 
cesses, and,  therefore,  always  indicative  of  the  former  presence  of 
vegetable  life,  but  this  is  going  too  far,  for  iron-ores  may  be  con- 
centrated in  other  ways.  But,  in  general,  it  may  be  said  that  such 
beds  are  presumptive  of  the  former  existence  of  organic  life. 

Constructive  Work  of  Animal  Life.  —  The  geologically  con- 
structive work,  which  animal  life  performs,  has  its  results  chiefly  in 
the  deposits  which  they  leave  behind  them.  By  far  the  greater 
part  of  these  deposits  is  composed  of  carbonate  of  lime,  a  small  and 
much  less  important  part  of  phosphate  of  lime.  The  deposit  of  car- 
Donate  of  lime  through  animal  life  takes  place  now,  and  has  taken 
place  in  the  past,  on  an  enormous  scale,  and  is  a  geological  process  of 
very  great  importance;  it  occurs  chiefly  in  the  sea,  and  is  most 
strikingly  illustrated  in  the  work  done  by  corals.  One  phase  of  it 
has  already  been  alluded  to  in  speaking  of  deposits  on  the  sea  floor, 
page  115. 

Coral  Reefs  and  Islands 

Corals.  —  These  are  small  animals  of  a  low  order  of  life.  The 
individuals  are  called  polyps,  are  simple  in  organization,  consisting 
chiefly  of  a  soft  sac-like  body  containing  a  stomach,  a  mouth,  and  a 
fringe  of  arms,  or  tentacles,  around  it,  with  which  they  capture  their 
food.  Further  details  respecting  them  are  given  in  Part  II.  One 
function  of  the  animal  is  to  extract  carbonate  of  lime  from  the 
sea-water,  and,  depositing  it  in  the  lower  external  part  of  the  body, 
to  build  up  a  stony  base  upon  which  the  animal  lives  and  flourishes. 
Living  together  in  colonies,  as  do  most  of  the  reef-corals,  this  stony 
base  grows  and  assumes  the  varied  shapes  seen  in  masses  of  coral, 
such  as  branching  forms,  plates  grouped  in  aggregates,  half  spheres, 
etc.  See  Fig.  145.  Coral  trees  or  the  "staghorn"  corals  may  be  15 
feet  high,  and  the  half  spherical  coral  heads  15  feet  across.  Such 
growths  support  an  enormous  number  of  individual  polyps.  While 
there  are  many  kinds  of  corals,  the  most  important  geologically  are 


ORGANIC   LIFE   AND    ITS   GEOLOGICAL   WORK 


183 


the  reef-building  ones.  These  are  not  found  everywhere  in  the  ocean, 
but  only  where  certain  suitable  conditions  prevail.  The  conditions 
demanded  are  as  follows:  a,  the  water  must  not  have  a  mean  tem- 
perature lower  than  68°  F.;  b,  the  water  must  be  clear  and  salt, 
free  from  the  products  of  land  waste ;  c,  the  water  must  be  shallow, 
not  over  240  and  preferably  less  than  150  feet  in  depth.  An  abun- 
dance of  food  is  also  necessary,  and  the  great  westward  tropical 
ocean  currents,  described  on  page  93,  seem  to  carry  this  and  produce 
very  favorable  conditions  for  corals  as  they  flow  against  the  eastern 


Fig.  145.  —  View  of  corals  on  a  reef  at  low  water.     Great  Barrier  Reef, 
Australia.     Saville  Kent. 

continental  shores.  Thus  the  eastern  shores  of  Africa,  Australia, 
and  Central  America  support  extensive  coral  formations,  whereas  on 
the  western  shores  of  these  continents  they  are  comparatively  rare. 
Coral  Reefs.  —  Corals  grow  upward  and  spread  laterally,  and  the 
stony  base  continually  increases,  branching  out;  or  the  coral  heads 
enlarge.  As  they  die,  the  lime  carbonate  base  is  left  and,  mingled 
with  the  growths  of  lime-secreting  algae,  the  shells  of  various  kinds  of 
shelled  animals,  the  tubes  of  worms,  and  the  bones  of  marine  ani- 
mals inhabiting  the  coral  thickets,  and  with  branches  and  pieces  of 
coral  broken  from  the  living  forms  above,  forms  a  constantly  ac- 
cumulating layer.  The  warm  sea-water  cements  the  coral  frag- 
ments together,  and  eventually  converts  the  deposit  into  a  white 
solid  limestone,  upon  whose  upper  surface  the  living  corals  grow 
and  flourish.  This  forms  the  coral  reef.  It  rises  until  its  surface 
is  just  below  the  level  of  low  tide,  only  occasionally  laid  bare  at  the 


184 


TEXT-BOOK   OF   GEOLOGY 


lowest  tides  for  short  periods,  for  corals  can  stand  exposure  to  the 
air  only  a  limited  time.  Over  it  the  waves  boil  and  break,  and  at 
its  outer  edges  the  reef-building  corals  thrive  best,  for  in  the  rush 
and  dash  of  the  waves  they  find  the  most  food  and  lime  in  the 
water,  and  here  the  latter  is  clearer,  more  aerated,  and  thus  fur- 
nishes more  of  the  needed  oxygen. 

In  addition  to  the  corals  all  such  reefs  have  many  other  forms  of 
animal  life  living  on  them,  and  some  of  these,  such  as  hydroids,  also 
contribute  by  lime  deposits  to  the  up-building  of  the  reef.  Even 
plants  (nullipores,  etc.)  add  their  share  by  secreting  lime  from  the 
sea-water,  and  it  seems  probable  from  recent  investigations  that 
they  have  been  a  much  more  important  factor  in  helping  to  build 
the  reefs  by  this  accumulation  than  had  been  previously  supposed. 

Coral  Islands.  —  The  term  coral  island  has  been  used  in  several 
ways,  and,  unless  strictly  defined,  is  liable  to  misinterpretation. 
There  are  a  great  number  of  islands  in  the  tropical  oceans,  especially 


Water-level,  High  Tide       Reef 
Ocea 


Fig.  146.  —  Section  through  coral  reef  and  island  upon  it.     After  Dana. 

in  the  Pacific,  which  for  the  most  part  are  of  volcanic  origin,  like 
Hawaii  for  example ;  a  few  are  high  islands  composed  of  rocks  which 
are  not  volcanic,  or  only  partly  so,  like  Fiji  and  New  Caledonia. 
These  are  more  or  less  surrounded  by  coral  reefs,  in  ways  which  will 
presently  be  described,  but  they  are  evidently  not  coral  islands. 
They  are  ordinary  islands  with  coral  reefs  about  them.  But,  by  the 
action  of  the  waves,  masses  of  coral,  often  large  coral  heads,  and 
blocks  of  reef-rock  are  broken  off  and  thrown  up  on  the  reef,  other 
fragments  and  coral  sand  fill  in  between  them,  and,  finally,  by  the 
beating  of  the  waves,  the  whole  mass,  resting  on  the  broad  platform 
of  the  slightly  submerged  reef,  is  compacted  and  rising  above  water 
becomes  an  island.  These  are  true  coral  islands,  consisting  of  the 
debris  from  organic  life,  the  material  composed  of  carbonate  of  lime 
secreted  from  sea-water. 

The  islands  thus  made  are  low,  usually  not  more  than  15  feet 
above  sea-level,  and  from  a  quarter  to  half  a  mile  wide,  though  often 
long  in  the  direction  of  the  reef.  They  are  usually  covered  with 
vegetation,  are  of  great  beauty,  and  sometimes  inhabited,  though 
their  lowness  subjects  them  to  the  danger  of  being  swept  by  the  sea 


ORGANIC   LIFE   AND   ITS   GEOLOGICAL   WORK  185 


in  times  of  heaviest  storms.    A  section  of  a  reef  with  a  coral  island 
upon  it  is  shown  in  Fig.  146. 

Classes  of  Coral  Reefs.  —  According  to  their  position  and  ar- 
rangement coral  reefs  have  been  divided  into  three  general  classes, 
which  are  known  as  fringing  reefs,  barrier  reefs,  and  atolls.  The 
characters  which  distinguish  them  are  as  follows: 

Fringing  Reefs.  —  In  the  shallow  water  around  any  existing  land 
where  the  conditions  are  right,  corals  grow,  and  gradually  build  up  a 
platform  to  sea-level,  forming  a 
bench   extending   outward   from 
the  land  edge  toward  the  sea,  as 
shown  in  Fig.  147.     The  width 
of  the  reef  seems  to  depend  on 
the  steepness  of  the  land  slope; 
if  this  is  great  the  reef  is  nar- 


row, if  gradual  it  may  be 
several  miles  wide.  Opposite 
streams  coming  from  the  land, 


Fig.  147.  —  Section  through  land,  and 
the  attached,  fringing  reef. 


the  reef  is  wanting,  since  these  produce  unfavorable  conditions  be- 
cause of  the  fresher  and  muddy  water. 

For  reasons  already  explained  the  corals  chiefly  grow  and  flourish  on  the 
outer  edge  of  the  reef.  The  seaward  slope  of  the  latter  is  very  steep.  As  ma- 
terial is  broken  off  by  the  waves,  and  rolls  down  this  slope,  it  gradually  be- 
comes compacted  by  deposit  of  lime  carbonate,  and  forms  a  rising  talus,  upon 

which,  eventually,  the  corals  grow  and  advance  the  reef  seaward. 

* 

Barrier  Reefs.  —  This  kind  of  reef  differs  from  the  fringing  one  in 
that  it  is  situated  some  distance  from  the  land,  with  a  stretch  of 
shallow  water  between,  forming  a  lagoon  or  channel.  Many  of  the 
high  volcanic  islands  of  the  Pacific  are  more  or  less  completely  gir- 
dled by  such  an  encircling  reef.  See  Fig.  148. 

Openings  or  breaks  exist  in  these 
reefs  sufficiently  deep  to  permit  the 
access  of  vessels  to  the  lagoon-chan- 
nels, which  thus  serve  as  harbors; 
the  channels,  which  have  an  aver- 
age maximum  depth  of  200  feet,  are, 
however5  often  too  shallow  for  navi- 
gation. The  barrier  may  be  from 
one  to  thirty  miles  from  the  land; 
often  they  support  islets  upon  them, 
which  may  be  wooded.  The  west 
coast  of  the  island  of  New  Cale- 
donia has  a  reef  of  this  character  Fig  148  _  gection  through  land  and  distant 
that  extends  for  400  miles,  while  the  barrier  reef  with  lagoon-channel  between. 


Part  of 


186 


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greatest  of  all  is  the  great  barrier  reef  of  Australia,  which  stretches  for  1200 
miles  along  its  eastern  side,  with  an  average  distance  of  20-30  miles  from 
the  mainland,  and  with  a  depth  of  100-300  feet  in  the  channel.  A  view  of  a 
portion  of  this  reef  at  very  low  water,  with  the  living  corals  growing  on  it, 
is  seen  in  Fig.  145. 

Atolls.  —  These,  like  many  barriers,  are  more  or  less  imperfectly 
ring-shaped  reefs,  but  without  any  island  within,  only  a  lagoon  of 
comparatively  shallow  water,  as  indicated  in  Fig.  149.  The  breadth 
of  the  ring  may  be  from  2  to  50  miles ;  generally  there  are  openings, 
usually  on  the  leeward  side,  affording  access  to  the  lagoon.  The 

depth  of  water  in  the  latter 

may  be  from  a  few  feet  up  to 
300,  but  averages  about  200; 
on  the  outside  the  reef  may 
descend  quite  sharply  thou- 
sands of  feet  toward  the  ocean 
floor.  Like  the  other  reefs 
mentioned  they  mav  support 

Fig.  149.  —  Section  and  plan  of  an  atoll.  _   .  * 

wooded  and  inhabited  islands 

upon  them.  A  view  of  an  atoll  is  seen  in  Fig.  150.  Such  atolls  are 
one  of  the  most  striking  features  of  the  Pacific  Ocean. 


Fig.  150.  —  View  of  an  atoll;  after  Dana,  from  an  old  picture. 

Origin  of  Barrier  Reefs  and  Atolls.  —  Fringing  reefs  require  no 
special  explanation;  their  origin  is  simple  and  may  be  understood 
from  the  description  of  them.  But  barriers  and  atolls  are  difficult 
to  understand,  both  from  their  form,  and  from  the  fact  that  they 
appear  to  rise  from  the  bottom  of  the  deep  ocean,  thousands  of  feet 


ORGANIC   LIFE   AND    ITS    GEOLOGICAL    WORK 


187 


below  the  limit  at  which  corals  can  grow.  How  then  did  these 
curious  structures  originate?  In  the  attempts  to  answer  this  ques- 
tion processes  have  been  invoked  which  make  the  explanations  im- 
portant from  the  bearing  they  have  upon  geological  problems  of 
great  significance. 

Subsidence  Theory  of  Darwin  and  Dana.  —  An  explanation  which 
for  a  long  time  received  general  acceptance  was  one  offered  by 
Darwin  and  elaborated  by  Dana.  According  to  this  hypothesis  the 
three  kinds  of  reefs  represented  successive  stages  in  a  continuous 
process  produced  by  gradual  subsidence  of  the  ocean  bottom.  The 


Fig.  151.  —  Section  showing  the  formation  of  barrier  reefs,  BR,  and  an  atoll  from  fring- 
ing reefs,  FR,  by  gradual  subsidence;  sea-level  remaining  permanent  but  assuming 
relations  A,  B  and  C. 

idea  involved  may  be  easily  understood  from  inspection  of  the 
diagram,  Fig.  151.  Around  some  island,  produced  perhaps  by 
volcanic  agencies,  corals  attach  themselves  and  grow,  forming  a 
fringing  reef  FR.  The  sea-level  is  supposed  to  be  at  A.  As  the 
island  gradually  sinks,  the  sea-level  remaining  the  same,  it  assumes 
with  respect  to  the  land  the  position  of  B.  Meanwhile  the  corals 
keep  building  the  reef  upward  and,  for  reasons  already  given,  most 
rapidly  on  the  outer  edge,  and  thus  the  fringing  reef  becomes  the 
barrier  BR.  By  a  continuation  of  the  process  the  island  disappears, 
and  the  barrier  becomes  an  atoll,  as  in  C.  It  is  understood  that 
the  rate  of  submergence  is  not  greater  than  the  upward  growth  of 
the  coral  reef.  There  are  many  facts  which  seem  to  confirm  this 
theory,  but  as  a  sufficient  explanation  for  all  kinds  of  coral  islands, 
objections  have  been  urged  against  it. 

What  seems  to  confirm  it  are  the  facts  that  all  gradations  between  the  three 
kinds  of  reefs  may  be  found,  and  that  there  are  positive  evidences  of  recent 
submergence  in  some  cases,  such  as  the  dredging  of  dead  and  drowned  corals 
from  the  reefs  at  depths  below  the  limit  of  coral  growth;  stone  houses  of  na- 
tives once  built  on  the  shore  and  now  surrounded  by  water,  etc.  A  boring  in  a 
typical  atoll  went  down  over  1000  feet  in  coral-reef  rock,  indicating  cor- 
responding subsidence  and  up-building.  The  topography  of  the  coast-lines  of 
the  volcanic  islands  with  barrier  reefs  also  shows  subsidence,  since  there  are 


188  TEXT-BOOK   OF   GEOLOGY 

no  sea-cliffs  cut  into  them,  the  streams  have  no  deltas,  but  drowned  river 
mouths,  and  the  shore-lines  are  very  irregular.  These  features,  and  others, 
relating  to  the  disposition  and  topography  of  the  groups  of  islands  with  coral 
reefs,  and  of  atolls,  have  recently  been  strongly  urged  by  Professor  W.  M. 
Davis  as  confirmatory  of  this  theory. 

The  objections  to  this  view  are  that  it  requires  a  general  subsidence  of  thou- 
sands of  feet  of  the  earth's  crust  over  such  a  vast  region,  some  20,000,000 
square  miles  of  the  sea  floor  of  the  Pacific,  as  well  as  subsidence  in  other 
oceans.  It  has  been  also  pointed  out  that  in  some  island  groups  there  are 
atolls  in  one  place  and  raised  coral  reefs  in  other  ones  not  many  miles  distant. 
Where  reefs  have  been  raised  they  are  found  not  to  be  over  250  feet  thick. 

Theory  of  Murray  and  Alexander  Agassiz.  —  This  attempts  to 
account  for  barriers  and  atolls  without  subsidence.  We  imagine 
first  a  platform  to  be  raised  in  some  way  from  the  ocean  floor  up  to 
the  required  depth  for  coral  growth.  This  might  happen  by  volcanic 
eruptions  and  up-building;  sometimes  the  volcanic  masses  would 
protrude  from  the  sea  and  form  islands;  sometimes  they  would  be 
not  very  high  above  sea-level  and  would  be  cut  away  by  the  waves 
to  shallow-water  platforms,  and  sometimes,  when  not  up  to  the  re- 
quired level,  they  might  be  raised  to  it  by  lime  deposits  from  the 
shells  of  marine  organisms.  Or,  as  Agassiz  suggests,  the  platform 
might  be  made  by  the  latter  method  alone.  Around  such  islands, 
and  on  the  platforms  thus  made,  coral  would  grow  and,  thriving  best 
on  the  outer  edges  of  the  reef,  the  latter  would  expand  and  move 
seaward,  advancing  on  the  talus  forming  at  its  front.  In  the  mean- 
time the  dead  portion  of  the  reef  left  behind,  bored  into  by  innumer- 
able organisms,  would  crumble  and,  partly  by  the  supposed  solvent 
action  of  the  sea-water  and  its  contained  carbon  dioxide,  and  partly 
by  the  scouring  of  currents,  would  be  gradually  removed,  and  in  its 
place  would  appear  the  channels  and  lagoons.  Thus  where  there 
was  an  original  island,  the  fringing  reef  would  move  away  and  be- 
come a  barrier;  where  only  a  submarine  platform  was  present,  it 
would  move  to  its  outer  edges  and  form  an  atoll. 

The  chief  objection  to  this  view  is  the  great  size  and  depth  of  many  lagoons 
and  channels,  30  miles  wide  and  200  feet  deep;  it  seems  improbable,  if  not 
impossible,  that  such  enormous  masses  could  be  removed  through  solution. 
Moreover,  the  study  of  atolls  shows  that  the  lagoons  tend  to  fill  up  rather 
than  to  deepen,  that  lime  carbonate  is  depositing  in  them  rather  than  being 
removed  by  solution.  It  also  does  not  account  for  the  general  evidences  of 
recent  submergence  mentioned. 

Rise  of  the  Water-level.  — Quite  recently  Professor  Daly  has  urged  the 
importance  of  a  rise  in  the  water-level  of  the  oceans  as  an  explanation  for  the 
origin  of  barriers  and  atolls,  an  idea  previously  advanced  by  Thos.  Belt,  Up- 
ham  and  others.  The  cause  of  this  increase  of  water  in  the  oceans  is  at- 


ORGANIC   LIFE   AND   ITS   GEOLOGICAL    WORK  189 

tributed  to  the  gradual  melting  of  the  vast  continental  ice-caps  which  covered 
in  a  recent  geological  period  the  North  and  South  Polar  regions,  and  extended 
down  into  what  are  now  temperate  latitudes,  as  will  be  described  more  fully 
in  a  later  place.  They  covered  millions  of  square  miles,  and  were  several 
thousand  feet  in  thickness.  The  gradual  accumulation  of  this  ice  lowered  the 
water-level  in  equatorial  regions,  and  its  gravitative  effect,  in  drawing  the 
oceanic  water  toward  it,  tended  to  lower  the  level  still  more.  Daly  calculates 
that  these  effects  lowered  the  level  of  tropical  seas  from  200  to  250  feet.  Owing 
to  the  colder  condition  of  the  earth,  and  thus  of  the  seas,  it  is  inferred  that 
coral  life  was  restricted  to  very  narrow  tropical  belts.  Elsewhere  the  islands, 
undefended  by  caps  and  belts  of  growing  coral,  were  exposed  to  the  erosive 
action  of  the  waves,  which  cut  wide  terraces  around  the  harder  and  larger  ones, 
while  small  ones,  and  those  of  softer,  less  compact  material  were  cut  off, 
forming  platforms  at  sea-level.  When  the  ice  melted,  and  the  seas  grew 
warmer,  the  corals  are  supposed  to  have  returned  to  these  islands;  on  the 
terraces  around  the  former,  growing  best  at  the  outer  edge,  they  formed 
barrier-reefs,  while  of  the  submerged  platforms  they  made  atolls.  Since  the 
reefs  grew  upward,  as  the  water  gradually  deepened  from  the  melting  of  the  ice, 
the  interior  lagoons  and  channels  were  formed,  and  these,  since  then,  have 
been  gradually  filling  up.  This  is  taken  to  account  for  the  general  depth  of 
250  feet  of  the  platforms  below  sea-level,  for  the  usual  depth  of  about  200 
feet  in  the  larger  channels  and  lagoons,  and  for  the  evidences  of  apparent  sub- 
sidence previously  mentioned. 

The  correctness  and  value  of  this  view  as  an  explanation  of  the  varied  fea- 
tures of  coral  islands  and  formations  mentioned,  and  its  general  bearing  on 
geology  can  only  be  ascertained  by  a  careful  comparative  study  of  varied 
groups  of  islands,  especially  of  their  topography,  and  of  the  relations  of  coral 
formations  of  past  ages.  It  also  depends  on  the  amount  of  water  withdrawn 
from  the  sea,  and  turned  into  ice,  and  the  data  on  which  this  in  turn  depends 
are  at  present  too  vague  to  enable  us  to  calculate  it  with  any  precision. 
The  amount  of  landward  cutting  by  wave  action  at  the  lowest  stage  of  water- 
level,  in  view  of  the  time  demanded,  would  in  some  cases  seem  excessive. 
It  is  in  one  respect  a  reversal  of  the  Darwin-Dana  view,  but,  like  that,  demands 
a  change  of  water-level. 

General  Explanation.  —  From  what  has  been  said  in  the  fore- 
going pages  it  appears  that  widely  divergent  views  have  been,  and 
still  are,  held  regarding  the  origin  of  the  peculiar  features  seen  in 
barrier  reefs  and  atolls.  Our  knowledge  in  several  directions  does 
not  seem  sufficiently  extensive  to  afford  a  general  explanation  which 
would  be  universally  accepted  at  the  present  time.  It  should  be 
pointed  out  that  the  views  of  Darwin  and  Murray  are  not  necessarily 
exclusive  of  each  other.  Barriers  and  atolls  could  be  formed  on 
either  assumption,  if  other  conditions  were  right,  and  outward- 
spreading  growth  and  submergence  might  be  occurring  simultane- 
ously. If  subsidence  were  the  only  factor  we  should  expect  a  great 
thickness  of  coral  rock  in  atolls,  whereas  borings  in  one  in  the  Pa- 
cific, and  quite  recently  in  Bermuda,  show  only  a  relatively  thin 


190  TEXT-BOOK   OF   GEOLOGY 

capping  of  it  on  the  volcanic  rock,  which  forms  the  main  masses 
rising  from  the  ocean  depths.  Quite  recently  Vaughan  has  called 
attention  to  the  fact  that  there  are  shallow-water  platforms  in  warm 
seas  which,  in  places,  are  devoid  of  coral  formations  of  barriers  and 
atolls,  like  the  one  surrounding  eastern  Australia,  which  shows  that 
the  platforms  antedate  the  settlement  upon  them  of  the  corals,  and 
are,  therefore,  independent  of  them.  He  also  shows  that  the  crescent 
or  ring  shape  of  atolls  may  be  due  to  the  effect  of  prevailing  winds 
and  currents ;  the  apex  of  the  crescent  pointing  toward  the  direction 
of  current  arrival.  Further,  recent  chemical  investigations  show 
that  the  water  in  the  lagoons  has  no  solvent  action  but  is  really  pre- 
cipitating lime  which  tends  to  fill  them  up.  The  suggestion  of  a 
change  in  sea-level  through  melting  of  ice- caps  may  prove  of  value 
as  one  factor  in  helping  us  to  understand  the  generally  uniform 
depth  of  water  on  the  shelves  which  support  barriers  and  in  the 
lagoons  of  atolls,  and  many  of  those  features  which  appear  obviously 
due  to  changes  of  water-level,  without  having  recourse  to  vast  sub- 
sidences of  the  floors  of  entire  oceans,  as  needed  by  the  Darwin- 
Dana  theory.  It  should  be  noted,  however,  that  other  factors  may 
produce  changes  of  water-level.  Thus,  over  the  vast  extent  of  the 
ocean  bottom,  which  covers  three-quarters  of  the  globe,  relatively 
slight  warping  movements  here  and  there  through  long  periods  of 
time,  causing  upward  and  downward  movements  over  local  areas, 
would  register  themselves  by  changes  of  water-level  on  the  shores. 
It  seems  probable  that  the  formation  of  the  coral-island  structures 
is  a  complex  one,  due  to  a  combination  of  several  agencies,  which 
operated  with  varying  intensities  in  different  places,  and  that  we 
cannot  advance  one  single  factor  which  will  cover  all  cases,  and 
afford  a  general  explanation. 

Lime  Carbonate  Deposits;  Limestone.  —  A  great  variety  of 
animals  living  in  the  sea  are  constantly  extracting  from  it  carbonate 
of  lime  for  their  own  uses.  In  the  sea-water  it  exists  as  the  soluble 
bicarbonate,  H2Ca(C03)2,  which  the  animals  convert  into  normal 
carbonate,  CaC03,  the  insoluble  form,  and  for  each  molecule  thus 
converted  one  of  carbon  dioxide,  C02,  is  set  free.  This  they  do  to 
produce  hard  parts  which  shall  support  or  protect  their  soft  parts. 
Familiar  examples  are  the  shells  of  molluscs,  such  as  clams,  oysters, 
sea-snails,  conchs,  etc.,  or  the  supporting  structures  of  corals.  There 
are  also  many  kinds  of  minute  free-swimming  animals  of  low,  or 
very  simple,  types  of  organization  living  in  the  upper  layers  of  sea- 
water,  which  have  protective  calcareous  shells.  One  important  group 
of  these  is  known  as  Foraminifera,  examples  of  whose  shells  are  seen 


ORGANIC   LIFE   AND    ITS    GEOLOGICAL   WORK  191 

in  Fig.  89.  Generally  they  are  not  larger  than  a  grain  of  sand.  It 
has  been  found  that  the  lime  deposits  in  the  sea  are  also  largely  due 
to  the  action  of  small  forms  of  vegetable  life,  varieties  of  algae,  float- 
ing in  the  upper  layers  of  the  water,  which  secrete  lime  from  it.  See 
also  page  112. 

These  varied  kinds  of  life,  small  though  they  may  be  in  the  indi- 
vidual, through  their  enormous  numbers,  and  working  through  long 
intervals  of  time,  have  produced  by  their  shells  and  other  structures 
deposits  of  carbonate  of  lime,  which  in  places  are  of  vast  extent. 
These  deposits,  accumulated  on  the  sea-floor,  according  to  their 
degree  of  compactness  and  other  characters,  as  we  are  able  to  de- 
termine them  after  they  have  been  raised  and  turned  into  land 
surfaces,  are  known  as  chalk  and  limestone.  Limestone,  therefore, 
is  a  sedimentary  deposit  of  carbonate  of  lime  made  by  organic  life  in 
the  sea. 

This  work  is,  perhaps,  most  conspicuously  seen  in  the  formation  of  the  coral 
reefs  previously  described.  A  coral  reef  might  be  likened  to  a  factory  for  the 
manufacture  of  limestone.  Along  with  the  corals  a  variety  of  other  organisms 
are  busily  at  work,  shelled  animals  of  different  kinds,  some  of  which  bore  into 


Fig.  152.  —  Serpuline  atoll.     Bermuda  Island.     These  structures  formed  in  shallow 
water  may  be  a  number  of  feet,  or  yards,  in  diameter  and  are  locally  called  "boilers." 

the  coral  rock  and  help  to  crumble  it;  worms  (Serpulse),  which  form  calcareous 
tubes  and  whose  colonies  may  form  miniature  atolls,  as  at  Bermuda,  Fig.  152, 
and  even  some  types  of  sea-weed  (nullipores) ,  which  secrete  carbonate  of 
lime  and  produce  coralline  structures,  while  Foraminifera  swarm  in  the  waters 
and  add  their  quota  of  shells  to  the  deposits.  Nor  is  the  deposit  confined  to 
the  reefs  and  their  immediate  neighborhood.  The  coral  rock,  broken  by  the 
waves  and  ground  up  to  fine  sediment,  is  distributed  over  the  sea-floor,  the 
water  being  muddy  with  it  after  heavy  storms  for  miles  away  from  the  reef. 
Such  fine  substance  consolidated  by  pressure  of  overlying  material,  and  by 
solution  and  re-deposition,  forms  compact  limestone.  The  rock  is  often 
dense  and  structureless,  and  without  fossil  remains  through  great  thicknesses, 
in  other  cases  filled  with  corals,  shells,  etc.  As  beds  of  limestone  are  found 


192  TEXT-BOOK    OF   GEOLOGY 

piled  up  in  thicknesses  of  hundreds  and  even  thousands  of  feet,  it  must  be 
inferred  that  their  formation  has  required  enormous  periods  of  time.  Agassiz 
has  estimated  that  it  would  take  about  1000  years  for  a  coral  reef  to  grow 
upward  40  feet.  It  is  especially  in  the  warm  waters  of  tropical  and  sub-tropi- 
cal seas  that  the  conditions  are  suitable  for  those  forms  of  life  which  deposit 
carbonate  of  lime,  and  where  it  therefore  accumulates  in  greatest  amount. 
Therefore,  the  presence  of  thick  beds  of  limestone  is  held  to  indicate  warm 
climate  in  that  region  at  the  time  of  their  formation. 

Shell  Limestones;  Coquina.  —  There  are  many  varieties  of  limestone,  depend- 
ing on  the  mode  of  formation.  Thus  in  some  there  are  abundant  remains  of 
some  particular  organism,  which  contributed  most  largely  to  the  deposit  in 
the  form  of  fossils,  and  which  gives  the  rock  a  peculiar  character.  It  may  be 
composed  almost  entirely  of  shells  with  fine  carbonate  of  lime  between 
them,  acting  as  a  cement.  Such  rocks  are  sometimes  called  "shell  limestones." 
A  light  fragile  rock,  consisting  of  shells  and  their  fragments  somewhat  com- 
pressed and  cemented,  now  forming  on  the  coasts  of  Florida,  is  known  as 
Coquina,  from  the  Spanish  word  for  shell. 

Chalk.  —  This  well-known,  soft,  slightly  coherent  rock  consists  of 
a  fine  calcareous  powder,  which  the  microscope  shows  to  be  largely 
composed  of  the  tiny  shells  of  Foraminifera  and  microscopic  plants 
(algae),  mingled  with  fragments  of  other  shells,  etc.  It  commonly 
contains  hard  nodules  of  siliceous  material  called  flint,  which  are 
supposed  to  represent  the  concentrated  hard  parts  of  certain  organ- 
isms which  secrete  silica. 

It  has  been  customary  to  consider  chalk  a  formation  produced  on  the  bot- 
tom of  the  deep  sea,  from  its  resemblance  to  the  calcareous  oozes,  or  muds, 
found  at  the  bottom  of  modern  oceans.  See  page  115.  It  would  seem,  how- 
ever, not  to  have  been  formed  as  a  deep-sea  deposit,  since  it  always  contains 
fossils  indicative  of  shallow  water,  as  well  as  skeletons  of  birds,  pterosaurs 
(flying  reptiles),  etc.  The  facts  in  most  cases  would  point  to  its  having  been 
formed  in  clear,  shallow,  and  warm  sea-water,  free  from  products  of  land 
erosion. 

Chemical  Precipitation  of  Lime  Carbonate.  —  Recent  investigations  by 
chemical  and  other  means  have  shown  that  the  ocean  water,  except  at  great 
depths  and  probably  on  the  surface  in  polar  regions,  is  saturated  with  car- 
bonate of  lime  in  solution.  It  is  therefore  obvious  that  if,  in  any  way,  the 
amount  of  carbonate  of  lime,  CaC03,  be  increased  by  concentration,  or  the 
capacity  of  the  water  to  contain  it  in  solution  be  diminished,  this  substance  will 
be  precipitated.  Thus  in  shallow  areas  an  increase  of  temperature  by  warm- 
ing the  water  may  cause  evaporation  and  loss  of  C02,  both  of  which  would 
produce  precipitation,  while  agitation  by  waves  would  also  cause  a  loss  of 
CO2  gas,  and  promote  precipitation.  These  facts,  which  have  been  pointed 
out  by  Vaughan  and  others,  show  that  lime  carbonate  must  be  thrown  down 
by  inorganic  means,  as  well  as  by  organic  life,  but  we  have  no  idea  yet  as  to 
the  quantitative  importance  of  the  process,  that  is,  of  the  amounts  which  may 
thus  be  precipitated. 

Dolomite.  —  Although  limestones,  when  first  formed,  consist  of  lime  car- 


ORGANIC    LIFE   AND    ITS   GEOLOGICAL    WORK  193 

bonate,  CaCO3,  in  process  of  time  they  have  been  in  places  more  or  less 
completely  converted  into  dolomite,  CaMg(CO3)2.  Even  if  not  pure  dolomite, 
if  they  contain  any  considerable  quantity  of  magnesia,  they  are  still  often 
referred  to  as  dolomite,  or  dolomite-limestone.  While  this  change  probably 
takes  place  in  greatest  amount  in  the  sea,  whose  waters  contain  magnesium 
salts  in  solution,  see  page  92,  and  especially  where  the  water  is  warm  and 
shallow,  it  apparently  may  occur  on  land  also,  caused  by  several  agencies,  the 
upward  movement  of  warm  waters  containing  magnesium  salts  for  example, 
but  is  more  restricted  in  extent.  Dolomite  is  a  more  stable  compound 
than  calcium  carbonate  and  forms  a  denser,  more  insoluble  rock. 

Phosphate  Deposits.  —  These,  while  not  making  geological  for- 
mations of  great  extent  and  importance,  are  of  interest  and 
of  great  commercial  value  from  their  use  as  fertilizers  for 
the  soil.  When  found  in  sedimentary  beds  we  ascribe  their  origin 
chiefly  to  the  calcium  phosphate  of  the  shells  of  some  marine  in- 
vertebrate animals  (brachiopods,  heteropods,  etc.) ,  and  the  bones  and 
excrement  of  vertebrate  animals,  concentrated  often  by  being  leached 
down  and  re-deposited.  Thus  they  occur  in  Tennessee,  Florida,  the 
Carolinas,  and  other  parts  of  the  South.  A  modern  illustration  of 
their  formation  is  seen  in  the  deposits  of  guano,  the  excrement  of 
sea-birds  in  certain  places  in  arid  regions,  as  on  the  west  coast  of 
South  America.  The  calcium  phosphate  it  contains  comes  chiefly 
from  the  bones  of  the  fishes  which  form  the  food  of  the  birds. 


CHAPTER  VIII 
IGNEOUS  AGENCIES;  VOLCANOES 

The  various  agencies  which  we  have  so  far  considered  as  modify- 
ing the  surface  of  the  earth,  such  as  the  atmosphere,  water  in  its 
forms  of  rivers,  seas  and  ice,  and  plant  and  animal  life,  derive  the 
energy  which  enables  them  to  move  and  perform  their  work  from  a 
source  exterior  to  the  earth:  from  the  sun.  For,  without  the  sun, 
these  movements  would  cease  and  the  earth's  surface  would  be  dead 
and  inert.  Toward  these  agents  the  earth  is  passive,  except  as  it 
adds  the  force  of  gravity  to  help  them  in  their  work.  An  exception 
to  this  principle  may  be  found  in  the  chemical  work  of  underground 
water,  otherwise  it  appears  to  be  a  general  one. 

We  have  now  to  consider  a  set  of  agencies  which  are  also  modi- 
fying the  earth's  surface,  whose  energy  on  the  other  hand  is  derived 
from  sources  within  the  earth  itself.  So  far  as  we  can  judge  they 
appear  to  be  due,  either  directly  to  the  interior  heat  of  the  earth,  or 
to  changes  going  on  within  which  produce  heat.  We  shall  describe 
first  the  results  as  seen  at  the  surface,  and  then  inquire  into  the 
possible  origin  of  them. 

When  we  regard  the  changes  going  on  within,  and  the  concomitant 
heat,  as  geological  factors,  the  processes  and  results  which  they  give 
rise  to  at  the  surface  may  be  grouped  as  follows: 

a.  Volcanoes  and  igneous  phenomena. 

b.  Hot-springs  and  fumaroles. 

c.  Changes  in  position  of  the  earth's  crust. 

d.  Earthquakes  as  a  result  of  c. 

Volcanoes 

General  Description.  —  Volcanoes  are  elevations  composed  of 
materials  collected  around  a  vent  through  which  they  have  issued 
from  the  earth's  interior  in  a  highly  heated  or  molten  condition. 
In  its  typical  aspect  a  volcano  is  conceived  of  as  a  steep  conical 
mountain  with  a  pit-like  crater  at  the  top,  from  which  issue  from 
time  to  time  gases,  ashes,  bombs,  and  flows  of  molten  rock  called 
lava.  The  ejection  of  material  is  termed  an  eruption,  and  volcanic 

194 


IGNEOUS  AGENCIES;   VOLCANOES 


195 


eruptions  are  to  the  human  mind,  perhaps,  the  most  impressive  of 
geological  phenomena,  from  the  immensity  of  the  forces  displayed, 
the  magnitude  of  the  results  achieved,  and  the  disastrous  conse- 
quences which  they  frequently  entail.  Volcanoes  are  apt  to  vary 
widely  from  the  typical  form  mentioned;  they  may  be  low  and  flat- 
tened, or  high  and  steep;  conical,  or  elongated  and  irregular  in 
shape ;  while  the  crater  may  be  at  the  top,  or  on  the  side,  of  variable 
shape,  or  even  wanting. 

In  size  volcanoes  may  vary  from  small  cones  one  or  two  hundred 
feet  high  to  those  which  form  some  of  the  loftiest  mountains  on  the 
globe.  Thus  certain  of  the  highest  peaks  of  the  Andes  are  formed  by 
volcanoes,  some  of  which  are  still  active,  as  Cotopaxi  in  Ecuador, 
19,600  feet  high,  with  a  crater  half  a  mile  in  diameter  and  1500  feet 


Fig.  153.  —  Mt.  Shasta,  Cal.     J.  S.  Diller,  U.  S.  Geol.  Surv. 

deep,  while  others,  like  Aconcagua,  23,000  feet,  and  Tupungato, 
21,500  feet,  on  the  border  between  Chile  and  Argentina,  and  Chim- 
borazo  (20,500) ,  in  Ecuador,  which  apparently  have  no  craters  and 
are  not  now  in  activity,  have  geologically  only  recently  become 
extinct.  These  are  built',  however,  upon  a  dissected  uplift,  or  plat- 
form, of  much  older  rocks,  above  which  they  rise  10,000-12,000  feet, 
but  in  the  case  of  the  Hawaiian  Islands  the  volcanic  piles  are  placed 
on  the  sea-floor,  some  14,000-18,000  feet  below  the  surface,  above 
which  the  highest  summits  project  about  14,000  feet,  thus  making 
the  whole  mass  some  30,000  feet  in  extreme  height.  In  the  United 
States  the  higher  peaks  of  the  Cascade  Range,  beginning  with 
Mt.  Shasta  (14,162  feet),  Fig.  153,  in  northern  California,  and  in- 
cluding in  Oregon  and  Washington  Mt.  Hood  (11,225),  Mt.  Adams 
(12,307),  Mt.  Rainier  (Tacoma)  (14,408),  and  Mt.  Baker  (10,703), 


196 


TEXT-BOOK  OF  GEOLOGY 


Fig.    154.  —  Eruption    of    Vesuvius,    April,    1906.     Seen    from    Boscotrecase.     The 
volcano  is  about  4000  the  ash  cloud  over  17,000  feet  high. 


IGNEOUS  AGENCIES;   VOLCANOES  197 

are  volcanoes,  which  are  now  quiescent,  or  have  recently  become  ex- 
tinct. Mt.  Etna,  on  the  coast  of  Sicily,  rises  about  11,000  feet  above 
sea-level,  and  the  diameter  of  the  base  of  the  conical  pile  is  about  30 
miles.  The  lower  slopes  are  gentle  and  studded  with  many  small, 
minor,  or  parasitic,  cones. 

Character  of  Eruptions.  —  At  volcanic  vents  three  things  may  be 
ejected,  gases,  liquids  consisting  of  molten  rock,  and  solid  material 
in  the  form  of  fragments,  and  the  nature  of  a  volcanic  eruption  de- 
pends largely  on  the  proportions  and  relations  of  these  three  things. 
If  the  eruption  is  violent  and  explosive  in  character  then  the  gases 
have  been  the  chief  factor  in  its  production,  and  solid  fragmental 
material  is  the  result ;  if,  on  the  other  hand,  it  is  quiet  in  its  opera- 
tion, liquid  rock,  or  lava,  is  the  main  product,  and  the  gases  play  a 
less  important  role.  We  may  thus  roughly  classify  volcanic  erup- 
tions into  those  which  are  explosive,  and  those  which  are  quiet  in 
nature.  When  we  attempt  to  classify  actual  volcanoes,  according 
to  this  difference  in  operation,  we  very  quickly  find  that,  although 
good  examples  of  both  types  may  be  found,  a  very  great  number, 
perhaps  the  majority,  are  intermediate  in  their  character,  that  is, 
they  sometimes  erupt  violently,  and  sometimes  give  rise  to  quiet 
flows  of  lava.  In  many  volcanoes  during  a  quiescent  stage  there 
appears  to  be  a  gradual  accumulation  of  pressure,  the  lava  rises  in 
the  conduit,  and  eventually  the  eruption  begins  explosively,  great 
quantities  of  gases  mingled  with  dust  and  stones  being  ejected;  the 
pressure  being  to  a  great  extent  relieved,  this  phase  is  succeeded  by 
a  quieter  one  in  which  the  lava  escapes  through  rents  in  the  cone  and 
forms  outflows  on  its  exterior. 

Explosive  Type.  —  In  the  most  extreme  form  volcanoes  of  this 
type  give  rise  to  sudden,  violent,  and  often  extremely  disastrous 
explosions.  Enormous  quantities  of  gas  are  suddenly  projected  into 
the  atmosphere,  so  thickly  mingled  with  comminuted  rock  (dust  and 
ashes),  as  to  form  vast -outrushing  and  expanding  clouds  of  dense 
appearance  and  dark  color.  See  Fig.  154.  The  greatest  known  ex- 
plosion of  this  character  occurred  at  Krakatoa,  a  volcano  in  the 
Strait  of  Sunda  near  Java,  in  August,  1883.  After  premonitory 
outrushes  of  gas  for  some  time,  the  great  explosions  occurred,  which 
blew  away  over  a  cubic  mile  of  material  from  the  volcano  into  the 
air  in  the  form  of  dust  and  ashes.  This  vast  dark  cloud  is  stated  to 
have  risen  17  miles  into  the  atmosphere,  completely  hiding  the  sun 
by  its  denseness  over  a  vast  area.  The  noise  of  the  terrific  detona- 
tions was  heard  for  more  than  150  miles,  while  the  disturbance  in 
the  atmosphere  was  registered  by  barometers  over  the  whole  world. 


198 


TEXT-BOOK    OF   GEOLOGY 


Huge  waves,  up  to  100  feet  above  tide,  were  generated  in  the  sea  and 
rushed  along  the  low-lying  coasts  of  Java  and  Sumatra,  sweeping 
far  inland  and  destroying  towns,  villages,  and  the  lives  of  nearly 
40,000  people;  they  were  perceptible  3000-4000  miles  away. 

In  May,  1902,  from  the  volcano  of  Mont  Pelee  on  the  island  of  Martinique, 
and  almost  simultaneously  from  that  of  the  Soufriere  on  St.  Vincent  in  the 
West  Indies,  after  small  premonitory  symptoms,  violent  explosive  eruptions 
took  place.  No  lava  was  outpoured,  but  the  intensely  heated  gases  were  so 
thoroughly  filled  with  incandescent  particles  of  rock  that  the  heavy,  fiery 


Fig.  155.  —  Fiery  cloud  of  Mt.  Pelee  descending  the  mountain  slope  into  the  sea. 
The  cloud  at  this  moment  is  7000  feet  high,  and  moving  forward  at  the  rate  of 
over  a  mile  in  1.5  minutes.  A.  Lacroix. 

clouds  not  only  rose,  but  acting  like  liquids,  rushed  down  the  mountain  slopes 
into  the  sea.  Destroying  all  life  in  its  course,  the  cloud  on  Martinique  en- 
veloped the  town  of  St.  Pierre  and  immediately  destroyed  it,  together  with 
its  30,000  inhabitants.  On  St.  Vincent  2000  people  perished  and  a  broad  tract 
of  country  was  devastated.  For  many  months  after,  Mont  Pele"e  continued 
to  eject  at  irregular  intervals  these  incandescent  clouds,  one  of  which  in  Fig. 
155  is  seen  rushing  into  the  sea. 

Intermediate  Type.  —  Probably  most  volcanoes  belong,  or  have 
belonged,  to  this  class.  In  them  an  eruptive  period  is 
likely  to  begin  with  explosive  activity,  manifested  by  the  pro- 
jection of  gases  in  great  quantity,  accompanied  by  solid  fragmental 
material,  bombs  and  ashes.  In  a  succeeding  phase  liquid  material 
issues;  it  may  be  projected  by  yet  issuing  gases,  or  it  may  break 
through  the  crater  walls  and  produce  outflows  of  lava,  sometimes  of 
great  volume.  Finally,  the  volcano  becomes  quiet,  its  energy  for 
the  time  being  exhausted;  the  lava  may  sink  down  in  the  conduit 
and  a  period  of  quiescence  intervene  before  the  next  eruption. 


IGNEOUS  AGENCIES;   VOLCANOES 


199 


While  this  sketches  in  a  general  way  the  succession  of  events  it  must 
not  be  supposed  that  all  volcanoes  of  this  class  are  alike  in  the  char- 
acter of  their  eruptions,  or  that  the  same  one  always  passes  through 
a  similar  set  of  phases  at  each  eruption,  for  there  is  great  variability 
in  these  respects.  The  main  point  is  that  volcanoes  of  this  kind 
both  exhibit  explosive  activity,  and  have  also  quieter  outflows  of 
liquid  lava. 

Vesuvius,  the  longest  and  most  studied,  and,  therefore,  the  best-known  vol- 
cano in  the  world,  belongs  in  this  class.  It  occupies  the  site  of  an  older  vol- 
cano, which  in  the  time  of  the  Romans  appeared  to  be  extinct,  for,  although 
they  recognized  its  nature,  they  had  no  traditions  of  its  having  been  active. 


Fig.   156.  —  Map  of  Vesuvius  and  vicinity. 

In  the  year  AJ>.  79,  the  volcano  again  became  active  in  eruptions  that  de-- 
stroyed  the  towns  of  Herculaneum  and  Pompeii  on  its  seaward  flanks.  A 
great  part  of  the  former  crater,  on  the  side  toward  the  sea,  was  blown  away, 
or  engulfed,  and  in  its  place  the  new  center  of  activity,  the  modern  Vesuvius, 
began  to  build  up.  This  has  continued  until  the  new  cone  is  about  4000  feet 
high.  Partly  enclosing  it  lies  the  sickle-shaped  ridge  of  Monte  Somma,  the 
remains  of  the  older  crater,  Fig.  156.  The  volcano  is  in  a  state  of  almost 
constant,  relatively  mild  activity,  with  irregular  periods  of  violent  eruption. 
The  last  great  eruption  occurred  in  1906,  see  Fig.  154.  From  the  nature  of  the 
material  composing  their  cones  it  seems  probable  that  the  great  volcanoes 
of  the  northwestern  United  States,  previously  mentioned,  and  now  quiescent 
or  extinct,  belonged  in  this  class,  as  well  as  the  active  ones  of  Alaska  and  the 
Aleutian  Archipelago. 

Quiet  Type.  —  These  give  rise  to  quiet  outflows  of  liquid  lava 
without  explosive  disengagement  of  gases  and  projection  of  solid  ma- 
terial as  dust,  ashes  and  bombs.  The  lava  in  this  case  is  very  hot 
and  possesses  great  liquidity.  There  is  a  more  or  less  constant  escape 
of  gases  from  it,  but  without  the  catastrophic  violence  of  the  pre- 
vious types.  The  best  example  is  found  in  Hawaii. 


200  TEXT-BOOK   OF   GEOLOGY 

The  island  of  Hawaii  consists  of  a  vast  mass  of  outpoured  lavas  surmounted 
by  several  cones,  Mt.  Kea,  now  extinct,  13,800  feet  high,  Mt.  Hualalai  (8,300), 
active  in  1801,  and  Mauna  Loa  (13,700),  now  active  and  some  of  whose  lava 
flows  have  been  50  miles  long.  On  the  eastern  slope  of  Mauna  Loa,  and  about 
20  miles  from  its  summit,  is  the  great  crater  pit  of  Kilauea,  of  a  rudely  oval 
form,  and  9  miles  in  circumference.  Its  rough  stony  floor  of  lava  is  a  cooled 
and  solidified  crust,  resting  on  the  top  of  the  vast  column  of  molten  rock, 
extending  down  to  unknown  depths  in  the  earth's  interior,  Fig.  157.  In  some 
places  it  is  not  crusted  over,  and  here  lakes  of  liquid  lava,  red  to  white  hot, 
and  boiling  from  the  escape  of  gases,  may  be  seen.  The  depth  of  the  crater 


Fig.  157.  —  Floor  of  the  great  crater  pit  of  Kilauea,  Hawaii.     To  the  left  the  view  is 
obscured  by  vapors  from  a  lava  lake.     J.  S.  Diller,  U.  S.  Geol.  Surv. 

floor  below  the  edge  of  the  rim  varies  according  to  the  height  of  the  lava 
column  on  which  it  rests;  after  a  discharge  it  may  sink  down  700  feet,  then 
through  a  period  of  years  the  lava  gradually  rises  until  it  stands  several  hun- 
dred feet  higher;  owing  to  the  increased  pressure,  and  perhaps  at  a  time  when 
the  elastic  forces  of  contained  vapors  are  unusually  great,  the  conduit  walls  are 
ruptured  and  outflows  of  lava  are  produced.  The  lava  column  then  sinks 
down,  carrying  the  crater  floor  with  it  to  a  lower  level,  until  equilibrium  is 
established.  Mauna  Loa  acts  in  a  somewhat  similar  way,  but  the  top  of  its 
lava  column  is  nearly  10,000  feet  higher  than  that  of  Kilauea.  The  outflows 
of  lava  are  more  apt  to  occur  through  the  flanks  of  the  mountain  than  through 
the  crater  rim;  they  sometimes  take  place  below  sea-level. 

Relation  between  Volcanoes  and  Magmas.  —  The  igneous  fluids 
of  the  earth's  interior,  which  give  rise  to  volcanic  action  and  vol- 
canoes, are  known  as  molten  magmas.  When  these  issue  out  on  the 
earth's  surface,  the  liquid  material,  and  the  rock  produced  by  its 


: 


IGNEOUS  AGENCIES;   VOLCANOES  201 

cooling  and  solidification,  are  called  lava.  It  must  not  be  supposed, 
however,  that  the  composition  which  a  solidified  lava  might  show,  if 
determined  chemically,  would  be  also  that/  of  the  magma  which 
yielded  the  lava.  For  the  deep-seated  magmas  contain,  in  addition 
to  the  mineral  substances  of  lavas,  great  quantities  of  gases,  espe- 
cially water  vapor,  which  are  held  in  them  under  pressure  in  a  kind 
of  molten  solution.  As  the  magma  rises  to  the  surface,  and  the  pres- 
sure is  relieved,  the  gases  escape,  usually  with  more  or  less  explosive 
energy,  and  give  rise  to  volcanic  activity.  Since  the  different  types 
of  volcanoes,  and  of  the  lavas  which  they  yield,  depend  in  large 
measure  on  the  magmas  producing  them,  it  is  necessary  at  this  point 
to  consider  the  nature  and  composition  of  these  molten  masses. 

Composition  of  Magmas.  —  As  indicated  above,  the  substances 
composing  the  earth's  magmas  may  be  divided  into  two  classes: 
a,  those  which  when  heated  are  volatile  in  their  nature  and  for  the 

ost  part  escape  as  vapors  and  gases,  such  as  water  vapor,  carbon 
dioxide,  hydrochloric  acid,  sulphurous  vapors,  etc.,  and  which  we 
shall  consider  in  more  detail  later;  b,  those  constituents  which  are 
non-volatile  and  remain  to  form  the  essential  ingredients  of  the 
solid  lavas.  These  latter  are  silica,  Si02,  and  the  oxides  of  six  metals, 
aluminum,  iron,  magnesium,  calcium,  sodium  and  potassium.  Silica 
in  variable  amount  is  always  present  in  the  magmas,  but  it  has  been 
found  by  chemical  means  that,  although  some  metallic  oxides  are 
always  present,  the  particular  kinds  may  vary  from  almost  nothing 
to  considerable  quantities.  Moreover,  there  is  a  kind  of  general  rule 
about  this;  without  going  into  details,  which  will  be  considered 
later  under  the  heading  of  igneous  rocks,  it  may  be  said  that  the 
magmas,  while  forming  a  complete  chemical  series,  may  be  divided 
into  two  classes,  one  in  which  silica  and  the  alkali  metal  oxides,  soda 
and  potassa  (Na20  and  K20) ,  become  more  and  more  predominant, 
and  the  other  in  which,  conversely,  lime  (CaO),  iron  oxides  (FeO 
and  Fe203)  and  magnesia  (MgO)  become  higher.  Lavas  of  the  first 
class  on  cooling  and  solidifying  may  crystallize  into  a  mass  of  min- 
eral grains  composed  chiefly  of  alkalic-feldspar,*  often  with  quartz, 
and  are  called  jelsites;  in  the  second  class  alkalic-feldspar  is  subor- 
dinate, and  quantities  of  lime,  iron  and  magnesia  minerals,  such  as 
iron-ore,  pyroxene,  lime-feldspar,  etc.,  are  formed;  lavas  of  this 
kind  are  termed  basalt..  Felsites  are  generally  light  colored,  while 
basalt  is  very  dark  to  black,  and  heavy  from  the  iron  minerals. 

These  characters  and  relations  may  be  summarized  in  the  follow- 
ing table: 

*  For  description  of  these  and  other  minerals  see  Appendix  A. 


202  TEXT-BOOK   OF   GEOLOGY 

{a.  Volatile  substances;  Gases  and  vapors,  e.g.  water,  CO2,  etc. 
b.  Non-volatile  substances;  Constituents  forming  solid  mate- 
rial, lavas. 

Chief  constituents  Chief  minerals  Resultant  rock 

{a.  Much  silica;  alumina,  alka-  Alkalic-feldspar,  quartz, 

lies.  etc Felsite  (light), 

b.  Less  silica;  lime,  iron,  mag-  Pyroxene,  lime-feldspar, 

nesia.  etc Basalt  (dark). 

Relation  to  Volcanic  Eruptions.  —  The  felsite  lavas,  or  rather 
the  magmas  which  produce  them,  are,  even  at  very  high  tempera- 
tures, up  to  over  2000°  C.,  thick  viscous  liquids,  apparently  from  the 
high  percentage  of  silica  they  contain,  the  amount  being  in  some 
kinds  as  much  as  75  per  cent  of  .the  whole.  For  this  reason  the 
contained  gases,  when  the  magma  rises  into  the  upper  part  of  the 
conduit  and  the  pressure  is  relieved,  escape  from  it  with  difficulty, 
and  often  with  violence,  giving  rise  to  explosive  eruptions.  Hence 
the  lavas  which  are  found  in  volcanoes  of  the  explosive  type  are 
apt  to  be  of  the  felsite  kind,  as  in  Mont  Pelee,  or  of  kinds  intermedi- 
ate between  it  and  basalt,  such  as  that  called  andesite,  which  occurs 
at  Krakatoa,  Cotopaxi,  etc.  On  the  other  hand  the  basaltic  mag- 
mas, with  about  50  per  cent  of  silica,  are  very  much  more  fusible, 
and  remain  quite  liquid  down  to  much  lower  temperatures,  probably 
1300°  C. ;  the  gases  escape  from  them  readily,  but  without  explosive 
violence,  as  illustrated  in  the  lava  lakes  of  Kilauea  in  Hawaii.  Thus 
quietly  eruptive  volcanoes  yield  basalt  as  a  lava. 

The  above  statement  indicates  the  general  rule;  it  does  not  mean  that 
basaltic  volcanoes  never  have  explosive  eruptions,  for  a  basaltic  magma  may 
become  cooled  in  the  conduit,  and  in  consequence  be  viscous,  and  thus  permit 
the  escape  of  magmatic  gases  only  with  difficulty  and  explosive  energy..,  Many 
examples  of  this  might  be  mentioned.  The  explanation  applies  chiefly  to  the 
two  extremes  and  indicates  what  is  probably  the  most  effective  cause  for  the 
explosive  and  quiet  types  of  volcanoes.  The  intermediate  type  of  volcano  may 
be  due  in  part  to  the  intermediate  kind  of  magma,  or  to  this  combined  with 
variations  of  viscosity  at  different  periods,  as  well  as  variations  in  the  supply 
of  gases. 

Products  of  Volcanoes 

Gases.  —  It  has  been  already  shown  that  the  products  yielded 
by  volcanoes  may  be  divided  into  three  general  classes,  gases  and 
vapors,  solid  fragmental  material,  and  liquid  rock  or  lava.  These 
may  be  now  considered  in  more  detail,  beginning  with  the  gaseous 
substances.  The  quantity  of  vapor  discharged  by  active  volcanoes 
is  immense,  and  is  indicated  by  the  height  and  volume  of  the  cloud 
with  which  many  eruptions  begin.  This  consists  of  the  dust  and 


IGNEOUS  AGENCIES;   VOLCANOES  203 

ashes  borne  aloft  by  the  uprushing  column  of  gases.  The  great 
quantity  of  vapor,  thus  discharged  into  the  atmosphere,  by  cond$- 
sation  may  give  rise  to  heavy  downpours  of  rain  in  the  vicinity  of 
the  volcano,  and,  owing  perhaps  to  the  friction  of  the  particles  and 
to  atmospheric  disturbance,  the  eruptions  and  rains  are  accompanied 
by  striking  electrical  displays  and  lightning.  Although  it  is  not 
directly  known  what  the  composition  of  the  gases  is  in  volcanic 
eruptions,  and  it  probably  varies  in  different  cases,  from  a  quantity 
of  indirect  evidence  it  is 'assumed  with  good  reason  that  it  is  chiefly 
water  vapor,  or  steam.  As  an  instance  of  the  quantity  of  water 
which  some  believe  is  discharged,  Fouque  estimated,  that  from  one 
of  the  subsidiary  cones  of  Mount  Etna,  there  was  discharged  in  100 
days  in  the  form  of  steam,  the  equivalent  of  over  460,000,000  gallons 
of  water. 

Quite  recently  some  geologists,  basing  their  opinions  largely  on  the  experi- 
ments and  ideas  of  Brun,  have  suggested  the  view  that  water  vapor  is  of 
minor  importance,  or  wanting,  in  volcanic  phenomena.  But  the  investigations 
of  Day  at  Kilauea  refute  this,  and  show  that  water  escapes  from  even  this 
quiet  basaltic  magma  in  considerable  quantities,  and  that,  by  means  of  iron 
pipes  suitably  placed,  it  could  be  collected  directly  from  it. 

In  addition  to  the  water,  the  different  kinds  of  gases  and  volatile  products 
exhaled  by  volcanoes  would  make  a  long  list.  Not  only  are  these  given  off 
from  the  vent  itself,  but  the  outflows  of  lavas,  for  weeks  and  even  months, 
after  their  extrusion  continue  to  emit  them  as  they  cool  and  harden.  It  ap- 
pears that  in  the  vents  and  from  the  hottest  lavas,  hydrochloric  acid,  hydroflu- 
oric acid,  and  even  hydrogen  are  given  off,  and  to  the  mixture  of  the  latter 
with  oxygen  and  its  sudden  combustion  are  sometimes  ascribed  the  explosions 
in  the  conduit.  Various  compounds  of  sulphur  are  emitted  by  some,  but  not  all, 
volcanoes,  such  as  sulphuretted  hydrogen,  H2S,  and  sulphur  dioxide,  SO2.  In 
declining  stages  of  activity,  and  in  less  heated  areas,  carbon  dioxide  appears 
to  be  one  of  the  chief  products.  Nitrogen  and  boric  acid,  H3BO3,  may  also 
be  mentioned.  Although  little  is  known  concerning  the  chemical  conditions 
of  these  substances  in  the  magmas,  the  knowledge  of  their  presence,  and  what 
we  have  so  far  been  able  to  learn  about  them,  are  of  great  value  and  interest 
in  their  bearing  on  important  problems  in  geology,  such  as  the  origin  of  hot- 
springs  and  geysers,  contact,  metamorphism,  and  ore  deposits,  as  we  shall  see 
later. 

Fragmental  Products.  —  These  are  the  materials  blown  into  the 
air  by  the  sudden  liberation  of  the  gases.  They  may  be  derived 
from  the  crust,  or  plug,  of  hardened  lava  left  in  the  upper  part  of 
the  conduit  after  a  previous  eruption,  from  rock  material  torn  from 
its  walls  or  from  lava  projected  from  the  upper  part  of  the  liquid  col- 
umn by  the  violent  expansion  and  expulsion  of  gases  from  the  magma 
due  to  relief  of  pressure  as  it  rises  to  the  surface.  In  the  latter 
case,  although  the  material  may  start  on  its  aerial  flight  in  a  liquid 


204  TEXT-BOOK   OF  GEOLOGY 

condition,  it  generally  hardens  in  its  passage  and  falls  in  solid  form. 
The  pieces  of  rock,  and  the  particles  of  magma  driven  upward  and 
solidified,  are  of  all  dimensions,  from  dust  so  fine  that  it  may  float 
in  the  atmosphere  for  several  years,  to  large  masses  of  several  hun- 
dred pounds  in  weight.  According  to  size,  they  are  roughly  classified 
as  follows:  pieces  the  size  of  an  apple,  or  larger,  are  called  bombs; 
those  the  size  of  a  nut  are  termed  lapilli  (meaning  little  stones) ; 
those  the  size  of  a  pea  are  volcanic  ashes,  while  the  finest  is  volcanic 


Fig.  158.  —  Volcanic  bomb,  Lipari  Islands. 

dust.  The  ashes  and  lapilli  are  frequently  spoken  of  as  volcanic 
cinders,  and  cones  made  of  them  as  cinder  cones.  It  should  be 
clearly  remembered,  however,  that  while  these  terms  are  used  to 
describe  the  appearance  of  the  products,  the  latter  are  not  the  result 
of  ordinary  combustion.  An  example  of  a  volcanic  bomb  is  seen 
in  Fig.  158. 

The  objects  described  above  are,  in  part,  composed  of  compact  solid  rock 
and,  in  part,  are  apt  to  have  a  spongy,  cellular,  or  vesicular  character.  This 
latter  is  due  to  the  fact  that,  while  the  major  part  of  the  gases  are  passing 
into  the  air,  and  carrying  the  fragments  with  them,  a  minor  part  are  expand- 
ing in  the  particles  of  liquid,  puffing  them  up  into  the  cellular  forms.  Al- 
though the  bombs,  lapilli,  and  most  of  the  ashes  fall  in  the  immediate  vicinity 
of  the  vent,  and  thus  help  to  build  up  the  cone,  the  dust  may  be  carried  long 
distances,  hundreds  of  miles  or  more,  by  the  prevailing  winds,  and  be  thus 
spread  over  an  immense  area.  Huge  quantities  are  discharged  in  great  erup- 
tions, amounting  to  many  millions  of  tons.  See  Fig.  154.  Such  dust  showers 
may  be  very  destructive  to  vegetation,  and  even  to  animal  life,  but  the  soil 
ultimately  yielded  by  them  is  very  fertile. 

Liquid  Material ;  Lavas.  —  In  volcanoes  whose  periods  of  erup- 
tion begin  .explosively,  the  liquid  lava  generally  issues  later,  after  the 


IGNEOUS  AGENCIES;   VOLCANOES  205 

vent  has  been  cleared.  The  cone  is  not  a  structure  of  great  strength, 
and  is  liable  to  be  ruptured,  or  fissured,  by  the  explosions  and  the 
pressure  of  the  lava  column,  and  hence  the  outflows  are  not  apt  to 
take  place  over  the  lip  of  the  crater,  but  to  issue  through  fissures  in 
the  side  of  the  cone.  It  may  even  happen,  especially  when  the  cone 
is  composed  of  cinders,  that,  unable  to  withstand  the  pressure,  one 
side  may  give  way,  allowing  a  flood  of  lava  to  rush  out  from  the 
breach  thus  made. 

The  appearance  and  character  of  a  lava  stream,  and  the  material 
produced  by  its  solidifying,  depend  on  several  things ;  on  the  chemi- 
cal nature  of  the  magma,  on  the  degree  of  viscosity  of  the  molten 
fluid,  and  the  extent  to  which  it  yet  retains  dissolved  gases.  On  the 
chemical  composition  will  depend  the  nature  of  the  rock,  whether  it 
will  be  a  light-colored  felsite,  or  a  black  basalt,  or  something  inter- 
mediate, as  previously  explained.  On  the  viscosity  will  depend  the 
rate  at  which  the  lava  will  flow,  the  distance  to  which  the  stream  will 
extend,  and  in  large  measure  the  appearance  its  surface  may  present. 
When  it  issues,  the  lava  is  red,  or  even  white  hot.  It  soon  cools  on 
the  surface,  darkens,  and  crusts  over.  If  very  viscous  the  under  part 
may  yet  be  in  motion,  the  crust  breaks  up  into,  a  mass  of  rough, 
angular,  jagged  blocks  of  rock,  which  are  borne  as  a  tumbling,  jost- 
ling mass  on  the  surface  of  the  slowly-moving  flow.  When,  even- 
tually, the  latter  comes  to  rest  and  hardens,  the  lava  field  produced 
is  extremely  rough  and  difficult  to  traverse.  See  Fig.  159.  Such 
lava  fields  in  Hawaii  are  called  aa  by  the  natives. 

On  the  other  hand  very  liquid  lavas,  like  those  of  Kilauea  and 
Mauna  Loa,  may  harden  with  much  smoother  surfaces,  which  ex- 
hibit, however,  curious  ropy,  curved,  wrinkled,  or  twisted  and 
billowy  forms,  as  seen  in  Fig.  160.  Lava  surfaces  of  this  kind  the 
Hawaiians  term  pahoehoe. 

Very  liquid  lavas  may  move  with  considerable  rapidity,  up  to, 
perhaps,  ten  miles  an  hour,  depending  on  the  slope,  Fig.  160;  as 
they  cool  and  become  viscous  the  motion  may  be  almost  indefinitely 
slow,  the  stream  creeping  onward,  possibly,  for  several  years. 

Sometimes  on  slopes,  after  the  lava  has  crusted  over,  the  liquid  portion  be- 
neath may  run  out  from  below,  leaving  beneath  the  hardened  surface  long  gal- 
leries, tunnels,  or  caves.  On  some  cones  the  natural  downward  drainage  may 
pass  into  these,  disappear  from  view,  and  issue  again  below  in  the  form  of 
springs.  This  may  be,  in  part,  the  cause  of  the  springs  around  Mt.  Shasta. 


I 


In  some  cases  the  magma,  or  lava,  ejected  is  too  viscous  to  flow; 
it  may  then  pile  up  in  a  great  dome  on  the  surface.    This  is  chiefly, 


206 


TEXT-BOOK  OF  GEOLOGY 


Fig.  159.  —  Rough  surface  of  an  advancing  lava  flow;  aa,  lava.    Vesuvius. 


r 


Fig.  160.  —  Flow  of  basaltic  lava  running  down  a  stream  bed,  the  water  of  which  is 
turned  into  steam.  This  lava,  if  cooling  as  seen,  would  have  the  pahoehoe  surface. 
Hawaii.  J.  S.  Diller,  U.  S.  Geol.  Surv. 


IGNEOUS  AGENCIES;   VOLCANOES 


207 


if  not  wholly,  confined  to  the  felsite  varieties  of  lava.  Domes  of 
lava  have  been  observed  in  central  France,  Bohemia,  Germany, 
etc.,  and  are  thought  to  have  been  formed  in  this  way.  They  prob- 
ably exist  elsewhere.  After  the  violent  eruptions  of  Pelee,  in  1902, 
had  cleared  this  orifice,  the  column  of  felsite  lava  that  filled  it  and 
hardened  into  rock  was  pushed  up  so  that  it  rose  like  a  vast  tower 
into  the  air  above  the  volcano,  until  it  attained  a  maximum  height 
of  1000  feet.  See  Fig.  161.  Gradually  it  crumbled  from  explosions 
of  gases  into  a  mass  of  blocks. 


Fig.  161.  —  Rock  tower  of  Mont  Pelee,  Martinique.     1000  feet  high.     A.  Lacroix. 

Effect  of  Contained  Gases ;  Vesicular  Lava.  —  That  the  lavas, 
even  after  their  issuance,  still  contain  dissolved  gases  is  abundantly 
shown,  not  only  by  the  clouds  of  steam  which  may  issue  for  weeks 
and  months  from  them,  but  also  by  the  structures  which  they  assume 
as  they  cool  into  stone.  Thus  the  upper  part  of  the  flow,  especially 
in  viscous  lavas  of  the  felsite  class,  may  be  so  puffed  up  by  the  in- 
numerable bubbles  of  vapor  in  it,  expanding  on  relief  of  pressure, 
that  it  may  assume  the  character  of  a  glassy  froth.  Such  rock  froth, 
which  is  usually  white  or  light  colored,  is  known  as  pumice,  or 
pumice-stone. 

In  more  liquid  lavas,  especially  those  of  the  basalt  class,  the 
bubbles  are  larger,  and  the  rock  has  a  spongy,  cellular,  or  vesicular 
character.  These  porous,  cindery,  or  slag-like  forms  are  called  veil- 


208  TEXT-BOOK   OF   GEOLOGY 

canic  scoria.     They  are  usually  dark  to  black,  or  reddish.     See 
Fig.  162. 

Pumice,  scoria,  and  vesicular  forms  are  characteristic  features 
of  the  upper  surface  of  lava  flows,  and  they  constitute  also  a  major 
part  of  the  coarser  fragmental  material,  such  as  bombs  and  lapilli, 
which  helps  to  make  the  cone.  See  page  204. 


Fig.  162.  —  Volcanic  scoria. 

Crystallization  of  Lavas ;  Glass  and  Stone.  —  After  lavas  have 
been  poured  out  and  have  solidified  they  usually  present  the  ordi- 
nary appearance  of  stone,  but  sometimes,  instead,  that  of  glass. 
The  reason  for  this  seems  to  be  as  follows.  If  the  liquid  is  not  too 
viscous  the  chemical  molecules  composing  it  will  have  capability  of 
motion,  and  will  arrange  themselves  into  definite  compounds,  that  is, 
will  crystallize  into  mineral  grains,  or  crystals.  It  may  be  that  the 
crystal  grains  are  large  enough  to  be  readily  seen  and  the  kind  of 
mineral  determined,  or  they  may  be  so  minute  that  the  lava  has  a 
homogeneous  appearance;  nevertheless  if  crystallization  has  taken 
place  the  lava  has  the  aspect  of  stone. 

On  the  other  hand,  if  the  lava  is  extremely  viscous,  or  quickly  be- 
comes so  through  rapid  cooling,  the  molecules  may  not  be  able  to 
arrange  themselves,  or  crystallize,  into  minerals,  and  the  mass  solidi- 
fies as  a  homogeneous  substance,  that  is,  as  a  glass. 

Thus  while  lavas  ordinarily,  in  hardening  into  rock,  adopt  a  stony 
aspect,  under  certain  conditions  they  may  assume  glassy  forms. 
This  occurs  chiefly  with  felsite  lava,  for,  as  previously  explained, 
this  kind  is  usually  the  more  viscous.  Volcanic  glass  is  called  obsid- 


IGNEOUS  AGENCIES;   VOLCANOES 


209 


tan;  less  commonly,  in  allusion  to  its  luster  and  appearance,  pitch- 
stone. 

In  some  cases  the  obsidian  is  pure  glass,  in  other  ones  a  mixture  of  glass  and 
crystals.  In  the  Yellowstone  Park,  Obsidian  Cliff  presents  a  section  of  volcanic 
glass  100  feet  thick,  which  has  cracked  into  columns  in  cooling.  Such  a  thick- 
ness of  purely  glassy  lava  is  unusual.  It  is  chiefly  on  the  edges  and  upper  sur- 
face of  lava  streams  that  these  glassy  forms  are  found.  Primitive  peoples, 
before  they  gamed  a  knowledge  of  metals,  made  much  use  of  obsidian  for 
making  knives,  arrow  and  spear  points,  etc.,  in  a  manner  similar  to  their  use  of 
flint. 

Varieties  of  Volcanic  Cones  and  Craters 

Kinds  of  Cones.  —  The  nature  of  a  volcanic  cone  depends  on  the 
kind  of  material  of  which  it  is  built.  If  composed  wholly  of  frag- 
mental  products  it  will  be  very  high  and  steep  in  proportion  to  its 


Fig.  163.  —  Cinder  cone,  showing  steep  angle  of  repose  of  lapilli.     Outline  as  given 
by  a  photograph  of  Mayon  volcano  in  the  Philippines. 

size  because  the  fragments  fall  around  the  vent  as  solid  pieces,  and 
the  angle  of  slope  is  that  of  the  angle  of  repose  for  such  broken  rock 
pieces.  Moreover,  as  lapilli  and  volcanic  ash  are  very  angular, 
rough  and  clinging,  slopes  of  40  degrees  may  be  attained  without 
sliding  of  the  accumulating  mass.  Cones  of  this  kind  are  often 
called  cinder  cones,  and  they  are  characteristic  of  volcanoes  of  the 


Fig.  164.  —  A  lava  cone,  to  show  contrast  with  Fig.  163.     From  the  Snake  River 

plain,  Idaho. 

explosive  class,  Fig.  163.  In  contrast  with  them  lava  cones,  formed 
entirely  by  quietly  outflowing  liquid  lavas,  like  that  of  Mauna  Loa, 
and  shown  in  Fig.  160,  are  necessarily  very  low  and  fiat  in  propor- 
tion to  their  size,  the  angle  of  inclination  being  less  than  10  degrees. 
See  Fig.  164.  These  belong  to  the  quiet  types  of  volcanoes.  Most 


210  TEXT-BOOK   OF  GEOLOGY 

volcanoes,  however,  and  this  includes  the  greater  part  of  the  largest 
ones  in  the  world,  are  of  the  intermediate  type  in  their  eruptions, 
and,  in  consequence,  their  cones  exhibit  a  form  intermediate  between 
those  just  described.  For  they  are  built  up,  sometimes  by  the  fall 
of  ashes  and  lapilli  when  they  are  explosively  active,  and  sometimes 
by  lava  flows  when  the  eruption  is  quieter.  This  is  the  character  of 
the  great  cones  of  the  Pacific  States,  Mts.  Shasta,  Hood,  Rainier, 
etc. 

In  the  larger  volcanoes  those  eruptions  which  often  break  out  on  their  lower 
flanks  give  rise  to  smaller,  subordinate,  or  "parasitic"  cones.  Mt.  Etna  is 
surrounded  by  over  200  of  these,  some  of  which  are  nearly  700  feet  high.  San 
Francisco  Mountain  in  Arizona,  an  extinct  and  partly  eroded  volcano,  exhibits 
a  number  of  such  minor  cones,  some  of  them  remarkably  well  preserved.  As 
an  active  volcano  grows,  the  earlier  parasitic  cones  may  be  buried  and  con- 
cealed under  later  accumulations,  or,  in  declining  stages  of  activity,  the  erup- 
tive energy  may  show  its  last  efforts  in  the  formation  of  them,  as  appears  to 
have  been  the  case  at  the  San  Francisco  volcano,  just  mentioned. 

Calderas;  Explosion  and  Subsidence  Basins.  —  The  term  cal- 
dera,  from  the  Spanish  for  caldron,  is  applied  to  crater-like  basins 
of  great  size,  especially  those  which  are  very  broad  as  compared  to 
their  depth.  The  name  is  taken  from  the  huge  pit  in  the  Canary 
Islands,  called  La  Caldera,  which  is  from  three  to  four  miles  wide 
and  faced  inwardly  by  lofty  cliffs  1500  to  2500  feet  high,  except  on 
one  side  where  the  wall  breaks  down  to  the  sea.  From  without,  at  a 
distance,  the  general  aspect  is  that  of  a  huge  cone  broadly  truncated. 
Many  examples  of  such  great  calderas  are  known  in  various  parts  of 
the  world,  and  a  study  of  them  has  led  to  the  view  that,  in  some 
cases,  they  have  been  caused  by  gigantic  explosions  which  have 
blown  away  a  great  part  of  the  original  cones  as  dust  and  ashes, 
leaving  the  calderas  to  mark  their  sites,  or,  perhaps  more  generally, 
that  they  have  been  produced  by  the  subsidence  of  the  column  of 
liquid  lava,  leaving  a  great  cavity,  which  the  central  part  of  the 
cone  subsided  into,  and  more  or  less  filled  up,  the  remnant  of  the 
cone  outside  making  the  caldera,  or  by  a  combination  of  these  two 
causes. 

Thus  Krakatoa  in  its  explosive  eruption  of  1883,  previously  alluded  to, 
appears  to  have  blown  away  a  good  part  of  the  original  volcano,  and  to  thus 
suggest  how  part  of  the  calderas  are  formed,  although  it  is  possible  that  an- 
other part  may  have  been  engulfed  by  the  subsidence  of  the  lava.  The  rem- 
nant left  may  mark  the  site  of  a  partial  caldera.  The  best  instance  of  a  cal- 
dera in  the  United  States  is  found  in  Crater  Lake  in  southern  Oregon.  This 
lake  occupies  a  caldera  at  the  summit  of  a  broad  sloping  volcanic  mountain 
in  the  Cascade  Range,  and  is  about  six  miles  long  by  four  broad,  2000  feet 
deep,  and  encircled  by  steep  cliffs  500  to  2000  feet  high,  Fig.  165.  An  island 


IGNEOUS  AGENCIES;   VOLCANOES  211 

in  it  made  by  a  small  but  perfect  cone  of  volcanic  material  indicates  a  feeble 
renewal  of  activity  after  the  principal  subsidence.  The  caldera,  if  emptied  of 
its  water,  would  appear  as  a  great  basin.  The  reaspn  for  believing  that  the 
caldera  was  formed  by  subsidence  of  the  lava  column,  and  engulfment  of  the 
greater  part  of  a  former  cone,  rather  than  by  explosion,  lies  in  the  existence  of 
glaciated  valleys  leading  up  the  outer  slope  of  the  mountain,  until  they 
abruptly  end  as  notches  in  the  cliff  wall,  and  in  the  absence  of  the  debris  which 
an  explosion  would  have  spread  over  the  adjacent  outer  slopes.  The  former 
mountain,  to  which  the  name  of  Mt.  Mazama  has  been  given,  is  conceived 
to  have  had  about  the  size  and  general  character  of  Mt.  Shasta,  and  during 
the  glacial  period  to  have  been  heavily  capped  with  snow  and  glaciers. 

It  may  be  noted  here  that  many  craters  and  calderas  of  extinct,  or  resting, 
volcanoes  are  filled  with  water,  giving  rise  to  lakes.  Several  of  the  circular 
lakes  of  Italy,  surrounded  by  volcanic  ejections,  like  Bolsena  and  Bracciano, 
are  regarded  by  some  geologists  as  marking  the  site  of  great  calderas. 


Fig.  165.  —  Part  of  the  basin  and  wall  of  Crater  Lake,  Oregon.     Note  the  small  cone 
within.     J.  S.  Diller,  U.  S.  Geol.  Surv. 

Explosion  Pits.  —  It  has  happened  in  some  cases,  where  volcanic  activity 
has  begun,  that  it  has  proceeded  no  further  than  the  initial  explosions  which 
have  forced  a  vent  through  the  country  rock.  The  material  blown  out  may 
make  a  low  slight  ridge  around  the  pit,  but  no  real  extensive  cone  is  built  up. 
Sometimes  volcanic  products,  such  as  pumice,  cinders,  etc.,  are  mixed  with  the 
fragments  of  the  country  rocks.  Such  basins  may  be  from  a  few  hundred  feet 
to  several  miles  in  width,  and  in  humid  regions  they  are  usually  filled  with 
water  and  form  lakes.  Some  of  the  best  examples  of  them  are  found  in  the 
region  west  of  the  Rhine  in  Germany,  known  as  the  volcanic  Eifel.  They  are 
there  called  maars  (German,  maareri),  like  the  Pulvermaar,  etc.  A  pit  which 
strongly  resembles  a  maar  exists  at  Coon  Butte  in  Arizona.  The  basin  sunk  in 
the  plain  is  about  3/4  of  a  mile  in  diameter  and  500  feet  deep.  The  presence 
of  meteoric  iron  in  and  about  it,  and  other  features,  have  led  to  the  view  that 
it  was  caused  by  the  impact  of  a  huge  meteorite,  and  is  probably  not  of  true 
volcanic  origin. 

Rebuilt  Volcanoes.  —  Not  infrequently  it  happens  that  after  a 
caldera  has  been  formed,  either  by  subsidence,  or  by  explosion,  or 


212 


TEXT-BOOK   OF   GEOLOGY 


both,  a  renewal  of  volcanic  activity  starts  building  up  a  new 
cone  within  it.  This  is  shown  on  a  small  scale  at  Crater  Lake  just 
mentioned,  but  one  of  the  best  examples  of  it  is  seen  in  Vesuvius, 
which  has  built  itself  up  in  the  old  crater  ring  of  Monte  Somma,  as 
explained  on  page  199.  From  this  rebuilding  within  the  older  crater 
wall  there  results  a  cone-in- crater  structure,  of  which  there  are  many 
examples.  The  vast  crater-like  pits,  which  are  so  common  on  the 
surface  of  the  moon,  frequently  show  this  arrangement,  suggesting 
an  analogous  origin  for  them.  It  is  conceivable  that  Vesuvius,  be- 
fore it  becomes  extinct,  may  go  on  increasing  in  size  until  the  old 
caldera  is  obliterated;  in  this  case  it  would  be  a  completely  rebuilt 
volcano. 

Structure  and  Dissection  of  Volcanic  Cones 

Structure  of  a  Composite  Cone.  —  If  a  column  of  molten  magma 
be  forced  upward  through  the  superficial  crust  of  the  earth  until  it 
reaches  the  surface,  the  relief  of  pressure  will  enable  it  to  commence 


Fig.  166.  —  Ideal  section  through  a  volcano.     The  dark  layers  in  the  cone  are  buried 
flows,  or  injected  masses. 

discharging  its  dissolved  gases  and  vapors.  It  may  be  that  the 
pressure  of  the  contained  gases  is  too  great  for  the  last  upper  layers 
of  bed-rock  to  restrain  until  the  magma  reaches  the  surface;  these 
rocks  may  be  blown  into  the  air,  and  a  vent  drilled  ahead  of  the 
rising  column  of  lava.  Arrived  at  the  surface,  an  outflow  may  take 
place  quietly,  or,  if  the  magma  is  too  viscous  for  this,  explosions  may 
continue  and  material  be  blown  upward.  By  the  falling  of  the 
fragments  the  cone  is  built  up,  somewhat  as  seen  in  the  diagram, 
Fig.  166.  The  pieces  cannot,  of  course,  fall  back  against  the  up- 
rushing  column  of  gases  and  cover  the  vent;  they  must  fall  outside 
of  the  latter,  the  heaviest  and  largest  first  and  nearest  to  it,  the 
smaller  and  lighter  later  and  farther  away,  the  distribution  of  the 
lighter  depending  much  on  the  wind.  Thus  the  cone  builds  as  a  cir- 
cular ridge  upon  whose  crest  is  the  heaviest  deposit  of  material, 


IGNEOUS  AGENCIES;   VOLCANOES 


213 


which  tends  to  roll  and  slide  both  ways,  outwardly  away  from  the 
center  and  inwardly  down  the  crater  toward  the  vent.  This  forms 
the  cone  and  crater,  and  certain  features  regarding  their  structure 
follow  as  a  sequence  of  this  mode  of  formation. 

Tuff  and  Breccia.  —  The  deposits  of  successive  eruptions  will  be 
marked  by  layers,  some  of  coarser,  some  of  finer  material,  in  each  of 
which,  if  not  composed  of  uniform-sized  particles,  there  is  a  grada- 
tion from  coarser  at  the  bottom  to  finer  at  the  top.  Thus  there 


Fig.  167.  —  Inclined  beds  of  volcanic  ash.     Part  of  a  former  cone  at  Trinchera,  Colo. 
W.  T.  Lee,  U.  S.  Geol.  Surv. 

arises  a  rude  stratification,  or  bedding,  the  beds  sloping  down  and 
out  from  the  crater  edge,  Fig.  167.  The  bombs,  lapilli,  and  ash 
composing  them  gradually  become  compacted  by  their  weight,  and 
by  the  infiltration  and  deposit  of  cementing  substances,  into  a  more 
or  less  friable,  porous  rock  called,  when  composed  of  the  coarser 
materials,  volcanic  breccia',  and  when  of  the  finer  dust  and  ashes, 
volcanic  tuff.  In  the  crater  the  fragments  are  larger,  often  large 
blocks  of  rock,  and  they  usually  form  a  tumultuous,  mingled  mass 
without  order  or  arrangement,  filled  in  with  finer  material,  the 
whole  called  volcanic  agglomerate. 

Lava  Flows  and  Dikes.  —  In  addition  to  the  beds  of  tuff  and 
breccia,  there  are  in  most  volcanoes  flows  of  liquid  lava  down  the 


214  TEXT-BOOK   OF   GEOLOGY 

outer  slopes  of  the  cone,  as  previously  described,  and  as  these  harden 
into  solid  rock  they  help  to  protect  the  softer  layers  of  tuff  and 
breccia  from  erosion,  and  to  give  strength  to  the  mass.  Since  the 
lava  rarely  overflows  the  crater,  but,  especially  in  high  cones,  breaks 
through  fissures,  or  vents,  lower  down,  these  latter  also  become 
filled  with  lava  which  hardens  into  rock.  These  rock-filled  fissures 
are  called  dikes,  and  like  ribs  they  also  serve  to  strengthen  the  struc- 
ture. Thus  a  vertical  section  through  a  volcano,  as  shown  in  Fig. 
166,  shows  a  central  core  of  magma  surrounded  by  beds  of  tuff  and 
breccia  mingled  with  flows  of  lava,  which  are  cut  in  a  general  radial 
direction  by  dikes.  This  discussion  gives  us  an  idea  of  the  general 
structure  of  a  typical  cone,  one  formed  by  the  intermediate  type  of 
volcano;  there  are,  of  course,  many  variations  from  this,  as  may  be 
inferred  from  what  has  been  previously  stated. 


Fig.  168.  —  Vesuvius  in  1906,  showing  trenching  by  ravines  in  the  ashes  after  the 
great  eruption.     Photo  by  F.  A.  Ferret.     Courtesy  of  Harper's  Weekly. 

Dissection  of  Volcanoes.  —  At  every  stage  of  its  existence  a  vol- 
cano is  subject  to  the  agencies  of  erosion  and  weathering,  which  tend 
to  cut  down  all  prominences  on  the  earth's  surface.  Its  height  and 
appearance  at  any  given  time  are  the  result  of  the  balance  between 
these  forces  and  the  upbuilding  one  of  vulcanism.  Even  active  and 
growing  volcanoes  are  commonly  trenched  and  scored  by  ravines 
and  gulches.  After  eruptions,  when  the  cones  are  covered  with 
fresh  deposits  of  dust  and  ashes,  the  latter  become  so  saturated  with 
water  from  the  rainfall  that  they  may  give  way  in  places  and  slide 
down  as  flows  of  liquid  mud,  leaving  ravines,  which  are  enlarged  by 
subsequent  storms.  See  Fig.  168. 

As  soon  as  a  volcano  becomes  extinct  the  ravages  of  erosion  are 
unchecked,  and  the  period  of  dissection  ensues.  The  lighter  tuffs 
and  breccias  are  carried  away  more  easily  and  rapidly,  the  harder, 


IGNEOUS  AGENCIES;   VOLCANOES  215 

more  compact  and  resistant  flows  and  dikes  of  lava,  and  the  parts 
protected  by  them,  more  difficultly  and  slowly.  It  is,  however,  often 
surprising  for  how  long  a  time  cones,  composed  of  mere  cinders 
loosely  piled,  will  resist  erosion  and  retain  their  form;  the  reason 
for  this  appears  to  consist,  partly  in  the  clinging  character  of  the 
rough  fragments,  and  partly  in  their  porosity,  which  allows  the 
rainfall  to  sink  through  without  causing  downwash. 

As  erosion  progresses,  the  mass  of  rock  formed  by  the  solidification 
of  the  magma  in  the  central  conduit  is  likely  to  be  brought  to  view, 
provided  the  magma  column  was  not  withdrawn  before  the  volcano 
became  extinct.  In  the  latter  case,  the  site  of  the  vent  will  probably 
be  marked  by  a  mass  of  agglomerate.  With  continuing  erosion  and 


Fig.  169.  —  A  volcanic  neck.     Black  shows  mass  of  volcanic  rock  through  erosior 
projecting  above  general  level  of  country. 

the  disappearing  of  the  cone,  the  central  rock  mass,  owing  to  its 
greater  resistance,  is  liable  to  form  a  decided  prominence,  and, 
even  when  erosion  has  finally  swept  away  all  external  evidences  of 
the  cone  and  bitten  deeply  into  the  underlying  rocks,  it  may  remain 
projecting,  a  monument  to  the  vanished  volcano.  A  rock  mass  of 
this  kind  filling  a  former  conduit  is  termed  a  volcanic  neck.  See 
Figs.  169  and  251.  Here  we  must  pause,  for  this  carries  the  dissec- 
tion of  the  volcano  to  its  very  root. 

Extinct  volcanoes  are  found  in  every  quarter  of  the  globe  and  in  great  num- 
bers of  regions  where  volcanic  activity  has  long  since  disappeared.  Every  stage 
of  dissection  Is  represented  among  them  from  cones  only  slightly  worn  to  those 
so  thoroughly  eroded  that  the  original  shape  has  been  entirely  lost,  but  whose 
central  rock  core,  outlying  concentric  masses  of  lavas,  tuffs  and  breccias,  and 
radial  dikes  still  plainly  show  their  former  existence.  The  general  region  of  the 
Rocky  Mountains,  once  a  theatre  of  active  vulcanism,  is  now  strewn  in  many 
places  with  the  wrecks  of  former  volcanoes.  A  good  example  of  this  is  found  in 
the  Yellowstone  Park  and  surrounding  country,  where,  as  we  shall  see  later,  the 
spark  of  vulcanism  still  lingers.  So  deeply  eroded  are  the  volcanoes  that  their 
remnants  now  form  a  region  of  most  varied  and  irregular  topography. 

Volcanoes  and  Deep  Masses  of  Magma.  —  In  the  preceding  par- 
agraph there  has  been  sketched  the  structure  of  the  volcano  down 
to  a  conduit  filled  with  magma  coming  from  below.  We  should  not, 


216  TEXT-BOOK   OF   GEOLOGY 

however,  leave  the  subject  of  volcanic  activity,  and  the  structures 
it  gives  rise  to,  at  this  point,  before  stating  that  volcanoes  are  only 
one  phase,  the  surface  manifestation,  of  the  general  movement  of 
masses  of  magma  from  unknown  depths  below  into  the  upper 
region  of  the  earth's  outer  shell.  Although  these  masses  of  magma 
may  attain  the  surface,  and  volcanoes  or  lava  flows  result,  often 
they  may  not,  but  remain  below  intruded  among  the  rock  layers  of 
the  shell,  and  there  cooling  and  solidifying,  give  rise  to  rock-bodies 
of  varied  shapes  and  of  sizes  from  a  few  feet  in  thickness  up  to 
miles  in  extent.  Since  a  proper  understanding  of  them  demands  a 
knowledge  of  the  structure  of  the  outer  shell,  they  are  best  treated 
under  the  heading  of  Structural  Geology,  where  they  will  be  found 
described  and  classified. 

Such  deep  bodies  of  magma  form,  then,  what  is  known  as  intru- 
sive masses  of  igneous  rock,  and  every  extinct  volcanic  conduit,  if  it 
could  be  traced  downward,  would  be  found  to  prolong  itself  into 
some  such  intrusive  rock  mass  below,  or,  if  active,  into  a  body  of 
magma  which  in  time  will  become  one. 


Life  and  Distribution  of  Volcanoes 

Age  of  Volcanoes.  —  We  have  very  little  knowledge  of  the  length 
of  time  represented  by  the  life  of  active  volcanoes.  Undoubtedly  it 
must  vary  greatly  in  different  individuals.  We  know,  for  example, 
that  Etna  has  had  the  same  general  character  for  the  last  2500 
years.  We  can  only  be  sure  that  the  piling  up  of  such  vast  masses 
of  material  as  are  represented  in  some  of  the  great  volcanoes  must 
have  required,  from  the  human  standpoint,  a  vast  lapse  of  time.  On 
the  other  hand,  we  know,  by  a  variety  of  considerations,  that  the 
present  active  volcanoes  are,  from  the  geological  standpoint,  recent 
affairs,  a  fact  which  indicates  to  us  in  one  way  the  great  length  of 
time  involved  in  the  past  history  of  our  earth. 

It  is  also  difficult  to  say  when  a  volcano  is  extinct,  because  long 
periods,  hundreds  of  years,  may  elapse  between  eruptions.  In  the 
Middle  Ages,  Vesuvius  had  been  so  long  dormant  that  its  crater  was 
filled  with  vegetation  and  gave  no  sign  of  life.  But  in  1631  it  be- 
came violently  eruptive,  and  has  since  been  intermittently  active. 

New  Volcanoes.  —  Within  the  period  of  recorded  human  knowl- 
edge a  number  of  volcanoes  have  begun  their  existence,  and  many 
of  them  are  still  active.  Vesuvius  is,  of  course,  the  most  noted  case 
of  this,  but  other  examples  are  seen  in  Jorullo  in  Mexico,  which 
came  into  being  Sept.  28,  1759,  in  the  midst  of  a  cultivated  plain, 


IGNEOUS  AGENCIES;  VOLCANOES  217 

and  is  now  about  4,300  feet  high,  and  in  Izalco  in  Salvador,  which 
began  in  1770,  has  been  almost  continuously  active  since  then,  and 
is  now  over  6,000  feet  high. 

No  well  authenticated  instance  of  a  volcanic  eruption  is  known  to  have  been 
witnessed  within  the  limits  of  the  United  States  proper  until  May  1914,  when 
explosive  eruptions,  which  have  since  continued  at  intervals,  began  at  Las- 
sen's  Peak  in  northern  California.  The  eruptions  have  been  chiefly  of  gases, 
ashes,  and  stones.  What  appears  to  be  the  latest  volcano  in  the  United  States 
is  found  in  a  cinder  cone  in  the  neighborhood  of  Lassen's  Peak.  See  Fig.  170. 
It  is  640  feet  high  above  its  base  with  a  crater  about  750  feet  in  diameter 


Fig.  170.  —  Cinder  Cone,  near  Lassen's  Peak,  Cal.     J.  S.  Diller,  U.  S.  Geol.  Surv. 

across  the  top  and  240  feet  deep.  After  the  cone  was  formed  a  large  flow 
of  basaltic  lava  burst  from  it,  and  nearly  filled  a  lake  upon  whose  border 
it  was  situated.  It  then,  apparently,  became  extinct.  From  the  trunks  of 
trees  still  standing,  which  were  killed  by  hot  ashes  at  the  time  of  its  early 
eruptions,  and  from  the  age  of  those  now  growing  on  these  ashes,  it  is  in- 
ferred that  the  volcano  began  somewhat  over  200  years  ago.  The  outflow  of 
basalt  was  considerably  later. 

Distribution  of  Volcanoes.  —  A  study  of  the  distribution  of  vol- 
canoes over  the  world  shows  that,  if  we  consider  the  present  active 
vents,  perhaps  500  in  number,  and  those  cones  which  have  suffered 
so  little  erosion  that  they  may  be  considered  dormant,  or  only  re- 
cently extinct,  of  which  there  are  several  thousand,  they  have  a 
general  tendency  to  be  grouped  in  long  lines  upon  the  earth's  sur- 
face. The  most  marked  one  of  these  is  the  great  zone  which  borders 
the  Pacific  Ocean;  it  passes  northward  along  the  Andes,  through 
Central  America  into  Mexico,  through  the  United  States  and  Canada 
to  Alaska,  then  along  the  Aleutian  chain  to  Asia,  and  turning  south- 


218  TEXT-BOOK  OF  GEOLOGY 

ward  through  Kamchatka,  Japan,  and  the  Philippines  it  crosses  the 
East  Indies,  and  by  various  island  chains  again  passes  into  the 
Pacific.  Certain  portions  of  this  belt,  like  the  Andes  and  the  Aleu- 
tian chain,  are  remarkably  linear  and  well  developed.  Another  great 
general  zone  has  an  east  and  west  direction,  from  Central  America 
through  the  West  Indies;  it  is  then  continued  through  the  Atlantic 


Fig.  171.  —  Map  showing  distribution  of  active  or  recently  extinct  volcanoes  in  the 
Eastern  Hemisphere.     On  S.  L.  Penfield's  stereographic  projection. 

by  the  Azores,  Cape  Verde  and  Canary  islands,  runs  through  the 
Mediterranean,  through  Asia  Minor  and  Arabia,  and  is  continued 
by  the  long  chain  of  the  East  Indies,  where  it  crosses  the  previous 
one,  out  into  the  Pacific.  This  linear  disposition  occurs  not  only 
on  a  large  scale,  affecting  series  of  volcanic  groups,  but  on  a  small 
one  as  well,  influencing  the  distribution  of  the  volcanoes  composing 
the  individual  groups.  It  may  be  studied  by  referring  to  Figs.  171 
and  172. 
Volcanoes  are  found  both  on  the  continents  and  in  the  oceans. 


IGNEOUS  AGENCIES;  VOLCANOES 


219 


the  true  oceanic  islands  seeming  to  be  entirely  volcanic.  Notably  in 
the  Pacific  there  are  great  numbers  of  them,  many  extinct  or  dor- 
mant, some  still  active,  and  here  again  in  many  cases  they  are 
grouped  in  lines,  and  stand  on  the  submarine  ridges  which  rise  from 


Fig.  172.  —  Map  showing  distribution  of  active  or  recently  extinct  volcanoes  in  the 
Western  Hemisphere.     On  S.  L.  Penfield's  stereographic  projection. 

the  ocean  floor.  From  the  fact  of  linear  arrangement  has  been  drawn 
the  important  deduction  that  volcanoes  are,  in  general,  situated  on, 
or  near,  lines  of  fracture,  folding,  and  weakness  in  the  earth's  crust. 

The  idea  of  the  connection'  between  volcanoes  and  fracture  lines  in  the 
earth's  crust  has  in  many  cases  been  pushed  too  far.  Undoubtedly  lines  of 
fracture  and  weakness  have  proved  favorable  sites  for  volcanic  action,  not 
only  for  a  time,  but  in  places  for  long-continuing  geologic  periods,  and  this 
has  greatly  influenced  their  origin,  situation  and  arrangement.  But,  on  the 
other  hand,  it  seems  clear  that  a  volcano,  or  a  group  of  them,  may  originate 
where  no  definite  connection  between  them  and  any  fracture  line  can  be 
shown  to  exist.  And  in  places  no  tendency  to  a  linear  arrangement  in  the 


220  TEXT-BOOK   OF   GEOLOGY 

group  may  be  seen.  It  appears  that  the  volcanic  forces  were  sufficiently 
powerful  to  find  an  outlet  without  needing  the  aid  of  a  fracture.  A  good 
example  of  this  may  be  seen  in  the  Highwood  Mountains,  a  group  of  extinct 
and  greatly  eroded  volcanoes  situated  on  the  great  plain  of  central  Montana. 
While  the  remaining  tuffs,  breccias,  lava  flows,  and  dikes,  composing  this 
group,  and  their  arrangement  and  attitudes,  show  clearly  the  cones  that  once 
existed,  erosion  has  dissected  them  so  deeply  that  the  shapes  of  the  cones 
have  been  lost,  the  central  conduits  now  filled  with  the  massive  rock  are 
exposed,  and  their  relations  to  the  sedimentary  bedded  rocks  through  which 
they  were  forced  laid  bare.  There  is  no  evidence  of  any  profound  breakage 
or  displacement  of  the  crust  on  which  they  could  be  placed,  nor  do  the  con- 
duits show  linear  arrangement. 

A  striking  instance  of  how  little  influence  favorable  disposition  of  surface 
topography  may  have  in  determining  the  site  of  volcanic  action,  which  in  the 
immensity  of  its  power  appears  to  disregard  such  minor  considerations  en- 
tirely, may  be  seen  at  the  Grand  Canyon  of  the  Colorado.  Uninfluenced  by 
its  5000  feet  or  more  of  depth,  volcanoes  have  broken  out  upon  its  very  rim, 
instead  of  in  its  depths,  and  their  lavas  have  flowed  down  into  it,  thus  show- 
ing also  their  recent  origin  compared  with  that  of  the  canyon. 

The  fact  that  almost  all  active  volcanoes  are  either  situated  in  the  sea,  or  in 
a  general  way  around  its  borders,  and  when  inland  are  in,  or  near,  lakes  has 
led  many  to  believe  there  must  be  a  necessary  connection  between  the  surface 
waters  and  the  cause  of  volcanic  activity.  This  question  will  be  considered 
later  in  the  discussion  of  the  origin  of  volcanoes. 

Submarine  Eruptions.  —  From  the  great  number  of  volcanic 
islands  in  the  sea,  the  real  oceanic  islands  being  of  this  nature,  it 
is  evident  that  in  times  past  tremendous  eruptions  and  vast  out- 
pourings of  lava  have  occurred  on  the  sea  floor.  The  volcanic  chain 
of  the  Hawaiian  Islands  is  an  example  of  this.  Instances  of  actual 
eruptions  beneath  the  sea  have  been  observed  in  a  number  of  cases 
and  recognized  by  the  issuance  of  vapors,  ashes,  etc.,  from  the 
water.  Thus,  in  1831  a  volcano  was  thrown  up  in  the  midst  of  the 
Mediterranean  Sea,  forming  a  new  island  called  Graham's  Island. 
Being  composed  of  light  cinders  it  was  soon  cut  off  by  the  waves  and 
reduced  to  a  shoal.  The  three  Bogoslov  volcanoes  of  the  Alaska- 
Aleutian  chain  formed  in  1796,  1883,  and  1906  are  other  examples. 
Such  eruptions  have  occurred  repeatedly  in  the  past  and  their  prod- 
ucts, mingled  with  sediments  from  the  land,  have  been  laid  down  as 
deposits  on  the  ocean  bottom,  as  seen  in  many  places  where  the  sea 
floor  with  these  deposits  has  since  been  raised  and  become  a  land 
surface.  Nor  do  these  volcanic  products  differ  in  their  essential 
characters  from  those  which  have  been  described  as  formed  by 
volcanoes  upon  the  land.  It  also  seems  probable  that  many  of 
the  cones  formed  beneath  the  sea,  and  thus  protected  from  ero- 
sion, are  of  great  age,  even  quite  old  from  the  geological  stand- 


IGNEOUS  AGENCIES;   VOLCANOES  221 

point,  and  have  served  as  the  base  for  coral  islands,  as  previously 
described,  page  184. 

Fissure  Eruptions.  —  In  several  regions  outflows  of  lava  have 
taken  place  on  such  a  gigantic  scale,  and  cover  such  widely  extended 
tracts  of  country,  that  it  is  commonly  believed  they  cannot  be  re- 
ferred to  the  outpourings  of  any  volcanic  cone,  or  group  of  vol- 
canoes. Moreover,  the  cones  from  which  they  might  have  come  are 
apparently  wanting.  It  is  thought  that  these  great  lava-floods  have 
issued  from  fissures  in  the  earth's  crust.  The  result  is  that  broad 
plains,  or  plateaus,  consisting  of  successive  level  sheets  of  basalt 
lava,  sometimes  interlaid  with  beds  of  tuff,  have  been  formed.  In- 
stances are  found  in  the  great  lava  fields  of  the  Columbia  and  Snake 
rivers  in  the  far  Northwest  of  the  United  States,  which  cover  from 
150,000  to  200,000  square  miles,  and  are  in  places  3,000  feet  deep, 
which  are  shown  on  the  geologic  map  accompanying  this  work;  in 
the  so-called  Deccan  traps  of  western  India  which  are  stated  to  be 
200,000  square  miles  in  extent  and  to  reach  a  maximum  thickness 
of  6,000  feet;  in  the  north  of  the  British  Isles,  which  in  part,  with  the 
outlying  island  groups,  appear  to  have  been  carved  by  the  sea  from  a 
great  basalt  plateau,  which  may  have  extended  to  Iceland.  The 
level  character  of  these  lava  sheets  is  evidently  due  to  the  extremely 
thin  liquid  nature  of  the  issuing  magma,  which  permitted  it  to  run 
for  many  miles  before  congealing.  Thus  in  Iceland,  a  lava  flow  has 
been  traced  a  distance  of  60  miles.  It  is  such  outpourings,  which 
occur  in  other  regions  as  well  as  in  those  mentioned,  which  exhibit 
to  us  the  grandest  effects  of  vulcanism.  It  is  a  conservative  esti- 
mate to  say  that,  since  a  comparatively  recent  geologic  period,  as 
much  as  500,000  cubic  miles  of  molten  material  have  been  trans- 
ferred from  the  inside  to  the  outside  of  the  globe  by  the  extrusive 
process  of  vulcanism,  most  of  it  in  the  manner  here  described. 

It  should  be  mentioned  in  this  connection  that  those  geologists  who  have 
most  closely  studied  the  great  lava  plains  of  the  Snake  River  do  not  believe 
that  the  molten  material  issued  from  an  extensive  system  of  fissures,  but 
from  various  vents,  like  those  of  ordinary  volcanoes  of  the  quiet  type,  some 
situated  along  the  sides  of  the  enclosing  mountain  chains,  others  on  the  plains 
themselves.  The  lava  craters  which  mark  the  site  of  the  vents  are,  for  the 
most  part,  so  extraordinarily  low  and  broad  as  to  escape  detection  in  a  gen- 
eral view,  and  are  only  found  by  closer  observation,  compare  Fig.  164.  This 
is  due  to  the  extremely  liquid  character  of  the  lava  which  spreads  out  in 
streams  50  miles  long  and  many  miles  wide.  Through  repeated  outpourings 
of  this  kind  the  previous  topography  was  buried  and  the  plains  produced.  It 
may  be  that  other  lava  plateaus,  referred  to  above,  were  made  in  a  similar 
way.  It  will  be  noticed  that  the  chief  distinction  between  the  two  views  is, 
that  in  the  one  just  mentioned,  the  vents  are  localized. 


222  TEXT-BOOK  OF  GEOLOGY 

Origin  of  Volcanoes 

General  Remarks.  —  So  far  as  regards  the  nature  of  volcanoes, 
the  character  of  their  eruptions  and  of  the  products  afforded  by 
them,  their  distribution  and  in  some  measure  their  life,  we  are  deal- 
ing with  ascertained  facts.  We  also  know  quite  clearly  the  reason 
for  the  different  kinds  of  eruption  and  the  varied  types  of  cones. 
But  when  we  seek  to  learn  the  cause  and  origin  of  vulcanism  we 
must  then  consider  the  depths  of  the  earth  itself,  about  which  we 
know  very  little.  We  are  led  from  facts  into  almost  pure  specula- 
tion, and  this  should  be  clearly  understood  by  the  student.  It  is 
evident  that  our  ideas  of  the  cause  of  volcanic  action  will  depend 
upon  those  which  we  have  concerning  the  nature  of  the  earth's  in- 
terior; what  has  been  learned  regarding  it  will  be  considered  in  a 
later  place.  There  are,  however,  certain  phases  of  it  which  may  be 
considered  here. 

Problems  of  Vulcanism.  —  Some  important  questions  that  arise 
when  we  seek  to  discover  the  cause  of  vulcanism  may  be  stated  as 
follows:  What  is  the  origin  of  the  heat  which  keeps  the  magmas  in 
a  molten  condition?  What  is  the  origin  and  history  of  the  molten 
magmas  which  come  to  the  surface?  From  how  deep  down  do  these 
magmas  come,  and  where  is  the  seat  of  vulcanism?  What  is  the 
origin  of  the  gases  and  vapors ;  have  they  always  been  contained  in 
the  magma,  or  has  it  absorbed  them  from  outside  sources,  and,  if 
so,  when  and  where?  And  finally,  what  causes  the  magma  to  ascend 
to  the  surface  from  depths  below  and  thus  give  rise  to  volcanoes? 
These  are  fundamental  problems,  most  of  which  our  knowledge,  at 
the  present  time,  is  too  limited  to  enable  us  to  solve.  Views  held  re- 
garding them  may,  however,  be  briefly  stated  and  discussed. 

Origin  of  the  Heat.  —  At  the  present  time  the  most  prevalent 
view  regarding  the  source  of  the  heat  demanded  for  the  molten 
magmas  is  that  it  is  original,  the  remains  of  a  globe  once  highly 
heated,  and  still  intensely  hot  in  its  interior.  Many,  however,  do 
not  share  this  opinion,  but  regard  the  heat  as  due  to  the  gradual 
contraction  and  compression  of  the  earth  through  the  force  of  grav- 
ity, a  process  which  should  cause  a  gradual  flow  of  heat  outward 
toward  the  surface,  and  by  its  concentration  in  particular  regions 
induce  melting  and  the  formation  of  masses  of  magma.  We  are  not 
yet  in  a  position  to  decide  definitely  between  these  views. 

It  has  been  urged  by  some  that  the  crushing  together  of  the  earth's  outer 
shell  through  contraction  must  generate  heat  on  an  enormous  scale.  It  is 
pointed  out  that  such  compression  and  crushing  have  taken  place  in  the  for- 


IGNEOUS  AGENCIES;   VOLCANOES  223 

mation  of  mountain  ranges,  as  we  shall  see  later,  and  it  is  inferred  that 
through  this  process  melting  has  occurred  and  volcanoes  have  been  made. 
There  are  two  objections  to  this  view.  The  first  is  that  volcanoes  are  often 
found  where  there  has  been  no  crushing  of  the  outer  crust,  or  at  least,  not 
for  an  immense  period  of  geologic  time  antedating  the  appearance  of  the 
volcanoes,  as  at  the  San  Francisco  Mountain  and  other  volcanoes  on  the  high 
plateau  in  Arizona.  The  other  is  that  the  folding  and  compression  of  the 
earth's  crust  which  makes  mountain  ranges  is  a  very  slow  process,  and  although 
great  quantities  of  heat  would  undoubtedly  be  generated,  it  has  not  been 
shown  why  it  would  not  be  as  rapidly  dissipated,  or  transformed  in  doing 
chemical  work.  How  could  it  become  accumulated  and  concentrated  suffi- 
ciently to  produce  melting  and  volcanoes?  For,  to  use  an  illustration,  what 
we  need  is  not  a  cask  of  warm  water,  but  a  cupful  of  boiling  water. 

Since  the  discovery  of  radio-activity,  and  the  researches  upon  matter  which 
it  has  induced  in  these  later  years,  have  shown  that  some  elementary  sub- 
stances are  disintegrating  and  breaking  down  into  other  substances,  as  for 
example,  uranium  into  radium,  helium,  etc.,  and  radium  into  helium,  lead,  etc., 
with  production  of  heat  in  notable  quantity,  it  has  been  assumed  by  some 
that  changes  of  this  nature  are  going  on  within  the  earth  and  that  in  this  way 
the  heat  necessary  for  volcanic  action  is  produced.  It  is  pointed  out  as  a 
proof  of  this  that  volcanic  regions  and  lavas  show  a  content  of  radio-active 
substances.  There  is  wide  diversity  of  opinion  on  the  subject  and,  at  present, 
this  view  has  not  advanced  beyond  the  speculative  stage. 

Origin  of  the  Magma.  —  This  is  evidently  closely  connected  with 
the  origin  of  the  heat,  as  just  discussed.  The  prevalent  view  is  that 
the  magma  is  a  remnant  of  the  original  molten  substance  of  the 
globe.  Those  who  hold  this  view  do  not,  however,  necessarily  claim 
that  it  has  always  been  in  a  liquid  condition.  In  melting,  rock 
material  must  expand;  if  sufficient  pressure  be  put  upon  it,  it  can- 
not expand  and,  therefore,  cannot  melt.  It  is  assumed  that,  with  the 
tremendous  pressure  reigning  in  the  earth's  depths,  the  material 
although  very  hot  would  be  solid,  but  should  relief  of  pressure  come 
in  any  place,  as  for  instance,  by  upward  buckling  of  the  earth's 
crust,  or  by  long  erosion  reducing  the  pressure,  or  by  both,  then 
melting  would  ensue  and  a  body  of  magma  would  be  formed.  A 
view,  once  held,  was  that -the  lavas  are  produced  by  the  fusion  of 
deeply  buried  sediments,  but  for  a  number  of  reasons  this  idea  now 
receives  little  credence. 

Another  theory  which  has  been  advanced  is  that  the  melting  of 
the  rock  material  is  due  to  the  issuing  of  superheated  gases,  squeezed 
out  of  the  deeper,  inner  portions  of  the  earth's  interior  by  its  gradual 
contraction  from  cooling.  They  are  supposed  to  melt  the  rocks  on 
their  upward  passage,  and  thus  give  rise  to  the  magma  and  to  vol- 
canic action.  The  difficulty  in  this  view  lies  in  the  enormous  quan- 
tities of  gases  that  would  be  required  to  melt  rocks  and  produce 


224  TEXT-BOOK  OF  GEOLOGY 

large  bodies  of  magma,  and  in  that  the  gases  under  pressure  tend  to 
go  into  solution  in  the  magma  and  become  inert.  And  so,  like  the 
origin  of  the  heat,  that  of  the  magma  must  for  the  present  be  con- 
sidered unsettled. 

Origin  of  the  Gases.  —  In  regard  to  this,  opinion  has  been  di- 
vided into  two  classes,  one  believing  that  the  gases,  as  indi- 
cated in  the  preceding  paragraph,  were  originally  contained  in  the 


Fig.  173.  —  Pavlof  Volcano,  Alaska.     G.  K.  Gilbert,  U.  S.  Geol.  Surv. 

earth,  which  has  been  gradually  losing  them,  and  another  that  they 
have  been  absorbed  by  the  magma,  especially  the  chief  gas,  water 
vapor,  from  surface  waters  descending  through  the  crust.  The 
former  appears,  from  several  considerations,  much  the  more  probable 
view,  and  is  the  one  mostly  held  at  this  time. 

The  fact  that  most  volcanoes  are  situated  in,  or  near,  the  sea  or  lakes  has 
been  considered  a  strong  proof  that  the  water-gas  contained  in  the  magma  has 
been  obtained  from  descending  surface  waters.  But  this  argument  when  ex- 
amined loses  its  force.  The  nearness  of  some  volcanic  chains  to  the  sea,  like 
those  of  North  and  South  America,  is  only  relative  to  the  size  of  the  con- 
tinental masses.  Actually  they  are  long  distances  inland;  in  South  America 
from  100  to  250  miles  and  this  includes  some  cones  still  active — like  Cotopaxi — 
which  are  not  near  any  inland  water  body;  in  North  America  from  30  to  100 
miles  or  more,  and,  although  these  are  mostly  extinct,  it  can  yet  be  shown  in 
many  cases  that  when  active  there  was  no  body  of  water  near  them.  And 
the  volume  of  steam,  from  even  small  volcanoes  at  periods  of  eruption,  is 
sometimes  so  enormous,  as  has  been  previously  shown,  page  203,  that  we 
cannot  attribute  it  to  ordinary  rainfall  or  surface  water,  leaking  downward 
through  the  rocks.  Moreover,  if  the  magma  obtains  its  water  from  surface 
supply,  it  must  do  so  in  the  very  upper  part  of  the  crust,  since,  as  already 
shown,  below  this  the  openings  which  permit  of  water  supply  and  circulation  in 
the  crust  are  closed  up.  But  several  considerations  show  us  that  at  such  shal- 
low depths,  far  from  absorbing  water,  the  tendency  of  the  magma  is  to  get  rid 
of  it  with  great  energy  and,  finally,  with  terrific  force.  And  if  the  magma  ab- 


IGNEOUS  AGENCIES;   VOLCANOES  225 

sorbed  such  enormous  quantities  of  water  and  converted  it  into  steam  it  is 
difficult  to  see  why  it  would  not  be  itself  cooled  off  and  solidified  in  the  pro- 
cess, and  thus  fail  to  reach  the  surface.  Hence,  we  conclude  that  certainly  the 
greater  part  of  the  water,  or  its  constituents,  and  perhaps  almost  all  of  it, 
which  comes  from  volcanoes,  was  originally  contained  in  some  form  in  the 
magma  and  brought  up  by  it  from  great  depths.  In  contrast  to  the  resident 
surface  water  of  the  earth  this  is  known  as  magmatic,  or  sometimes  less 
happily  as  juvenile  water.  It  appears  probable  that  the  other  volatile  con- 
stituents previously  mentioned,  page  204,  are  in  great  part,  if  not  entirely, 
likewise  original. 

Cause  of  Ascension.  —  What  causes  the  magmas  to  ascend  we 
do  not  know,  but  the  fact  that  the  situation  of  the  great  volcanic 
chains  is  on  those  belts  along  which  movement  and  disturbance  of 
the  lithosphere  (the  earth's  outer  shell  of  rock)  have  taken  place  is  a 
significant  one.  For  these  are,  apparently,  zones  of  weakness  in  the 
lithosphere,  and  have  thus  afforded  favorable  positions  for  the  up- 
ward movement  of  the  magma,  and  its  escape  to  the  surface.  As 
will  be  explained  more  clearly  later,  the  lithosphere  is  divided  into 
great  blocks,  or  segments,  and  these  have  in  times  past  moved  up,  or 
down,  with  respect  to  one  another.  It  is  often  noticeable  that  where 
one  of  these  great  blocks,  measuring  hundreds  or  thousands  of 
square  miles  in  area,  has  sunk,  this  has  been  attended  with  uprise 
of  magma,  outflows  of  lava,  and  commonly  with  volcanic  action.  In- 
stances are  found  in  the  depressed  tracts  which  form  the  great  Rift 
Valley  of  East  Africa,  the  valley  of  the  Rhine,  the  region  of  the 
Christiania  Fiord  in  South  Norway,  and  the  sunken  sandstone  area 
of  Connecticut  and  Massachusetts. 

All  this  suggests  that  the  upward  movement  of  the  magma  is  due 
in  some  way  to  variations  of  pressure  induced  by  changes  of  position 
of  the  segments  into  which  the  lithosphere  is  divided,  although  we 
do  not  know  just  how  to  explain  its  mode  of  operation.  Some  hold 
that  the  gradual  contraction  of  the  solid  earth  through  gravity, 
aided  by  other  processes,  causes  the  magmas  to  rise.  The  old  idea 
that  the  earth  has  a  hot,  liquid  interior,  and  that  the  downward 
pressure  of  the  contracting  cold  and  solid  lithosphere  forces  this 
liquid  out,  and  thus  gives  rise  to  volcanoes,  has  been  completely  dis- 
proved by  a  number  of  considerations,  and  is  no  longer  held.  The 
independent  eruptions  of  adjacent  volcanoes  in  the  same  group,  and 
the  fact  that  the  lava  column  in  Mauna  Loa  stands  10,000  feet  higher 
than  that  in  Kilauea,  only  20  miles  away,  are  disproofs  of  this  view, 
while  others  will  be  mentioned  later. 

As  to  the  seat  of  volcanic  action,  or  the  point  from  which  the 
magma  may  be  considered  to  move  on  its  upward  way,  we  know 


226  TEXT-BOOK   OF   GEOLOGY 

nothing.  One  conjecture  we  can  make  is  this.  From  the  study  of 
earthquakes,  and  for  other  reasons  as  we  shall  see  later,  it  appears 
that  the  earth  consists  of  an  outer  shell  —  lithosphere  —  whose 
specific  gravity  is  about  2.7,  and  this  shell  is  thought  to  be  not  of 
very  great  thickness,  and  to  change  gradually  into  a  denser  core. 
The  specific  gravity  of  the  earth  as  a  whole  is  5.6,  hence  the  inner 
core  must  be  much  denser  than  the  outer  rock-shell  —  the  lithos- 
phere. The  specific  gravity  of  much  of  the  lavas  erupted  by  vol- 
canoes is  not  much  over  3.0,  while  that  of  a  large  portion  is  consider- 
ably less.  Hence  it  would  appear  that  the  material  forming  the 
lavas  was  probably  not  derived  from  far  within  the  inner  core,  and 
this  may  determine  the  seat  of  action  as  being  not  many  miles  deep. 

Hot-Springs  and  Fumaroles 

Introductory.  —  In  the  foregoing  description  of  volcanoes  it  has 
been  shown  what  an  active  role  gases  and  vapors,  especially  water 
vapor,  play  in  their  eruptions.  But  long  after  a  volcano  has  ceased 
to  be  active  and  has  passed  into  a  dormant,  or  dying,  stage  these 
volatile  substances  continue  to  issue  from  its  crater,  or  from  its 
flanks,  or  even  from  places  in  the  surrounding  country.  In  the  same 
way  thick  beds  of  extruded  lavas  continue,  often  for  years,  to  exhale 
steam  and  other  vapors.  And,  as  we  shall  show  later,  it  has  often 
happened  that  large  and  small  bodies  of  magma  have  penetrated 
into  the  outer  shell  of  the  earth,  without  attaining  the  surface  or 
forming  volcanoes,  and  these  in  solidifying,  like  the  lavas,  have 
given  off  quantities  of  the  same  volatile  substances  which  work 
upward  through  fissures  and  pores  in  the  rocks.  It  is  now  proposed 
to  describe  the  class  of  phenomena  produced  by  such  emanations  at 
the  surface.  They  may  appear  as  vapors,  or  in  liquid  condition; 
the  former  may  be  considered  under  the  general  heading  of  junta- 
roles,  the  latter  under  hot-springs. 

Fumaroles.  —  This  word,  which  is  derived  from  a  Latin  verb 
meaning  to  smoke,  is  applied  to  fissures,  or  holes,  in  the  rocks  and 
soils,  from  which  steam  and  other  heated  vapors  escape  with  more 
or  less  force.  Although  steam  is  the  most  common  substance,  other 
vapors,  such  as  hydrochloric  acid,  hydrogen  sulphide,  carbonic  acid, 
etc.,  also  occur.  When  the  fumaroles  give  off  sulphurous  vapors 
they  are  often  termed  solfataras,  from  the  Italian  word  for  sulphur. 
It  is  noticeable  that  in  the  hottest  fumaroles  the  acid  gases  are 
prominent ;  in  those  less  hot,  various  other  volatile  compounds,  often 
hydrogen  sulphide,  which  decomposing  gives  rise  to  deposits  of 


IGNEOUS    AGENCIES;    VOLCANOES  227 

sulphur;  while  as  the  fumaroles  become  cooler,  or  cold,  carbon  dioxide 
becomes  the  chief  emanation.  Although  different  regions  vary  in 
the  proportion  and  nature  of  the  products  exhaled,  this  general  rule 
seems  to  hold,  not  only  in  time  but  in  space,  so  that  whether  one 
considers  the  decline  of  activity  at  a  given  center,  or  travels  away 
from  an  active  one,  the  change  from  hot  acid  vapors  to  cooler  carbonic 
acid  exhalation  is  noticed.  A  view  of  several  fumaroles  is  seen  in 
Fig.  174. 


Fig.  174.  —  General  view  of  the  Norris  Geyser  Basin,  showing  hot-springs  and  fuma- 
roles, and  the  white  siliceous  deposit  of  geyserite  from  them.     Yellowstone  Park. 

The  Solfatara  near  Naples  had  its  last  eruption  in  1198;  since  then  it  has 
been  in  the  condition  of  discharging  steam  mingled  with  sulphur  vapor,  and 
this  has  given  rise  to  the  use  of  the  term  solfataric  stage  when  volcanoes  be- 
come quiescent,  or  are  dying.  Some  of  the  great  cones  of  the  Northwest,  like 
Mt.  Shasta,  appear  to  be  in  a  dying  solfataric  stage.  In  the  Yellowstone  Park 
the  solfataric  condition  is  still  active  and  fumaroles  abound  in  many  places. 
Although  at  times,  and  in  places,  the  steam  given  off  in  fumaroles  can  be 
ascribed  to  magmatic  origin,  it  is  often  increased,  or  even  surpassed  or  re- 
placed, by  descending  surface  water  being  vaporized,  either  by  contact  with 
hot  rocks,  or  by  the  heated  volcanic  exhalations.  This  is  probably  the  case 
in  the  Yellowstone  Park. 

The  closing  condition  in  which  carbon  dioxide  gas  is  given  off  is  found  in 
numerous  places  where  volcanic  activity  still  abounds,  and  in  many  where  vul- 
canism  has  long  since  died  out.  It  may  happen  that  it  issues  directly  from 
the  ground  as  a  gas  spring,  and  such  occurrences  are  known  as  mojets.  Being 
heavier  than  air  it  may  collect  in  still  weather  in  depressions  near  the  vent, 
and,  since  it  is  colorless,  tasteless,  and  odorless,  such  pools  of  the  gas  may 
prove  deadly  traps  for  animal  life,  by  suffocating  such  creatures  as  enter  them. 
This  is  illustrated  in  "Death  Gulch"  in  the  Yellowstone  Park,  and  in  simi- 


228  TEXT-BOOK  OF  GEOLOGY 

lar  places  elsewhere.  But  the  gas  is  far  more  likely  to  encounter  ground- 
water  on  its  upward  way  and  thus  give  rise  to  carbonated  springs,  which  pass- 
ing through  limestone  may  deposit  carbonate  of  lime.  See  page  165.  Some 
hold  that  the  carbon  dioxide  is  derived  from  the  decomposition  of  the  lime- 
stone, CaO-CO2,  and  this  may,  in  some  places,  and  in  part,  be  true,  but  in 
other  places  it  can  with  good  reason  be  referred  to  a  magmatic  origin. 

Hot-springs.  —  In  volcanic  regions  hot-springs  are  likely  to  occur. 
We  may  attribute  their  origin  to  different  causes ;  they  may  be  due 
to  descending  surface  waters  being  heated  by  coming  in  contact 
with  hot  rock  masses  below,  or  by  hot  magmatic  vapors  passing  into 
them,  and  their  returning  in  this  heated  condition  to  the  surface. 
When  they  occur  in  active  or  quiescent  volcanic  regions,  as  in  the 
Yellowstone  Park,  they  are  probably  due  to  a  varying  combination 
of  the  causes  mentioned  above.  Warm  springs,  with  temperatures  up 
to  120°  F.,  also  occur  in  regions  where  no  direct  evidence  of  connec- 
tion with  volcanic  activity  or  intrusion  of  magmas  can  be  shown;  in 
this  case  they  are  probably  deep,  or  fissure  springs  (see  page  157) , 
and  the  water  has  been  warmed  by  contact  with  rocks  whose  tem- 
perature has  been  raised  by  mechanical  means,  such  as  crushing,  or 
by  chemical  changes  going  on  within  them. 

It  is  impossible,  in  certain  regions,  to  tell  in  the  case  of  hot-springs  and 
fumaroles  how  much  of  the  water  (and  steam)  is  surface  and  how  much  is 
magmatic  in  origin.  It  probably  varies  in  different  cases.  Hot-springs  in  the 
rainless  arid  interior  of  some  deserts  have  been  regarded  as  mostly  of  mag- 
matic origin.  The  proof  that  magmatic  emanations  have  passed  into  such 
waters  is  found  in  the  presence  in  them  of  such  substances  as  sulphur,  arsenic, 
boric  acid,  etc.,  in  quantities  and  under  conditions  which  show  that  they  could 
not  have  been  leached  out  from  the  original  rocks  of  the  country.  In  the  Yel- 
lowstone Park  it  is  probable  that  the  greater  part  of  the  water  is  of  surface 
origin,  which  becomes  heated  by  contact  with  lavas  still  hot,  and  returns  in 
this  condition,  but  there  are  good  indications  that  magmatic  vapors,  once 
very  active,  still  play  a  subordinate  part. 

While  there  are  various  types  of  hot-springs,  dependent  on  their 
temperature,  substances  in  solution,  etc.,  the  most  interesting  are 
boiling  springs  and  geysers.  Warm  carbonated  springs  depositing 
travertine  have  been  already  described,  page  167. 

Boiling  Springs.  —  Actively  boiling  springs  are  a  feature  of  many 
volcanic  regions.  A  considerable  number  of  them  occur  in  the 
Yellowstone  Park,  especially  in  the  different  geyser  basins,  see  Fig. 
175.  They  exhibit  various  gradations  from  pools  which  are  hot,  but 
rarely  boil,  or  else  quietly  simmer,  into  types  which  boil  strongly 
and  steadily,  and  some  even  more  or  less  violently  and  with  some- 
what explosive  energy,  interrupted  by  short  periods  of  repose.  The 


IGNEOUS  AGENCIES;   VOLCANOES 


229 


latter  form  transitions  to  the  geysers  mentioned  beyond.     So  long  as 
the  supply  of  water  is  sufficient  to  enable  the  spring  to  have  an  over- 


Fig.  175.  —  "The Devil's  Punch-Bowl."  A  hot-spring  boiling  in  the  cup-like  deposit 
of  geyserite  it  has  formed.  The  opening  is  several  feet  in  diameter.  Upper 
Basin,  Yellowstone  Park.  W.  H.  Weed,  U.  S.  Geol.  Surv. 


Fig.  176.  —  Mud  Volcano,  Waiotapu  Valley,  New  Zealand. 

flow  it  remains  limpid,  and  it  usually  has  a  deep  blue,  or  green  color, 
but,  if  the  evaporation  through  boiling  is  equal  to  the  inflow,  the 
water  is  more  or  less  turbid  from  particles  of  disintegrated  rock,  and 


230  TEXT-BOOK  OF  GEOLOGY 

eventually  becomes  a  mass  of  boiling  mud.  The  mud  may  be  white, 
or  variously  tinted  yellow,  red,  purplish,  or  black  by  oxides  of  iron 
and  manganese,  and  such  hot-springs  are  called  "paint-pots,"  "mud- 
pots,"  etc.  The  mud  as  it  increases  in  amount  may  become  so 
thick  and  viscid  as  to  prevent  regular  ebullition,  and,  owing  to  the 
accumulating  steam  pressure,  action  may  happen  spasmodically 
and  with  some  violence,  the  mud  being  thrown  into  the  air  and  about 
the  vent,  where  it  collects  in  considerable  masses,  see  Fig.  176. 
These  are  known  as  mud  volcanoes,  or  mud  geysers.  They  usually 
mark  a  declining  stage  of  activity  in  the  life  of  a  hot-spring. 

Geysers.  —  This  term,  from  an  Icelandic  word  meaning  to  gush, 
is  applied  to  certain  hot-springs  which  at  intervals  spout  a  column  of 


Fig.  177.  —  Basin  of  the  Oblong  Geyser,  partly  empty,  but  filling  with  water  after  an 
eruption.  The  rounded  masses  are  deposits  of  white  silica,  geyserite.  Yellow- 
stone Park.  Haynes  photo. 

hot  water  and  steam  into  the  air.  Depending  on  the  size  of  the 
geyser,  and  its  special  peculiarities,  the  column  of  water  may  be 
only  a  few  feet  high,  or  from  that  up  to  several  hundreds;  the  erup- 
tion may  last  a  few  minutes,  or  several  hours ;  the  quantity  of  water 
discharged  may  be  small,  or  be  many  thousands  of  gallons;  the  jet 
may  play  steadily  and  continuously  straight  up,  or  be  fitful,  be 
composed  of  minor  jets,  or  be  thrown  in  inclined  directions.  The 
interval  between  eruptions  may  be  a  definite  one  of  a  number  of 
minutes,  or  hours,  or  it  may  be  quite  irregular,  and  several  days 
may  elapse  between  them.  Each  geyser  has  in  these  ways  its  own 


IGNEOUS  AGENCIES;   VOLCANOES 


231 


peculiarities.  As  they  are  special  kinds  of  boiling  springs  they  are 
not  common  and,  so  far  as  known,  appear  to  be  confined  to  three 
regions,  the  Yellowstone  Park,  Iceland  and  New  Zealand. 


Fig.  178.  —  Lone  Star  Geyser  in  eruption,  showing  cone  of  geyserite. 

Park.     Haynes  photo. 


Yellowstone 


Fig.  179.  —  Old  Faithful  Geyser  in  action.     The  jet  of  water  is  100  feet  high,  at  this 
time.     Yellowstone  Park. 

Some  geysers  consist,  at  the  surface,  of  a  basin,  which  may  be  several  feet  to 
a  number  of  yards  long,  and  broad,  and  rather  deep.  The  sides  and  edges  of 
the  basins  are  usually  beautifully  ornamented  by  the  deposits  of  silica  de- 


232 


TEXT-BOOK   OF   GEOLOGY 


scribed  beyond,  and  terminated  at  the  bottom  in  tubes  or  fissures  leading  to 
the  heated  depths  below;  this  type  is  illustrated  in  Fig.  177.  The  tubes  and 
basins  are,  except  after  eruptions,  filled  with  water  at,  or  near,  the  boiling 
point.  In  other  types  the  geysers  by  their  deposits  have  built  up  mounds,  or 
cones,  of  silica,  from  a  foot  or  two  to  several  yards  high,  which  form  upward 
continuations  of  the  pipes.  See  Fig.  178.  Of  the  Yellowstone  geysers  the 
most  celebrated,  perhaps,  is  the  one  known  as  "Old  Faithful,"  which  for  many 
years  after  its  discovery  had  a  very  regular  interval  between  eruptions  of 
about  65  minutes,  Fig.  179.  It  is  now  becoming  more  irregular.  This,  and  the 
decline  of  activity  in  other  geysers,  or  springs,  does  not  mean  any  immediate 
diminution  of  thermal  action  in  this  region,  only  changes  going  on  in  the 
underground  system  of  pipes  and  fissures  which  conduct  and  supply  the  hot 
water.  Altogether  there  are  several  dozen  fine  geysers  in  the  park,  while  the 
number  of  hot-springs,  fumaroles  and  thermal  vents  of  various  kinds  amounts 
to  several  thousand.  It  is  a  fact  not  easily  explained  that  geysers  have  been 
found  only  in  felsite  lavas. 

Cause  of  Geyser  Action.  —  The  intermittent  eruptive  action  of 
geysers  depends  on  the  relation  between  pressure  and  the  boiling 
point  of  water,  as  was  pointed  out  by  Bunsen  in  connection  with  the 
great  geyser  in  Iceland.  The  boiling  point 
of  water  under  the  ordinary  pressure  of  the 
atmosphere  at  sea-level  is  212°  F.;  increase 
of  pressure  raises  it,  a  decrease  lowers  it. 
Thus  the  boiling  point  at  the  bottom  of  a 
column  of  water  will  be  raised  by  the  pres- 
sure of  the  superincumbent  layer  above  it; 
as  shown  in  Fig.  180,  it  will  gradually  rise 
as  we  follow  the  tube  from  the  surface 
downward.  If,  however,  the  cavity,  or  fis- 
sure, be  large  and  open,  the  heated  water 
below  will  rise,  convection  currents  will  be 
established,  mingling  the  water,  so  that  it 
will  have  nearly,  though  not  quite,  the  same 
temperature  in  different  parts  of  the  cavity, 
and  a  regular  boiling  spring  will  result.  But 
if  the  tube  be  long,  narrow,  tortuous,  or 
constricted,  convection  will  be  prevented,  or 
restrained,  and  the  water  must  boil  in  dif- 
ferent levels  at  different  temperatures  cor- 


250 


Fig.  180.  —  Diagram  to  nius-  responding  to  the  pressures.    Suppose  at  a 
trate  conditions  necessary  point  230°  in  the  figure  the  boiling  point 

for  geyser  action.  111111  f 

is  reached,  bubbles  of  steam  are  formed, 

the  column  of  water  above  is  raised  a  little  by  the  expansion,  the 
bubbles  of  steam  rise,  into  the  cooler  liquid  above  and  collapse, 


IGNEOUS  AGENCIES;   VOLCANOES  233 

the  column  of  water  settles  back  with  jarring,  thudding  sounds 
commonly  heard  before  eruption.  The  temperature  of  the  water 
will  gradually  rise  until  it  is  just  about  at  the  boiling  point  for  each 
level  corresponding  to  its  depth  and  pressure.  Finally,  when  a  suffi- 
cient volume  of  steam  is  formed  in  the  lower  parts,  the  expansion 
will  cause  some  of  the  water  in  the  basin,  or  cone,  at  the  top  to 
overflow,  this  lowers  the  pressure  throughout  the  tube,  and  the  water 
at  each  level,  being  now  heated  above  the  boiling  point  for  the 
diminished  pressure,  will  immediately  flash  into  steam,  and  a  min- 
gled column  of  steam  and  hot  water  will  be  driven  roaring  out  of 
the  pipe  into  the  air.  After  the  eruption  is  over  the  system  fills 
again  by  inflow  of  ground-water  through  the  fissured  rock,  and  the 
process  is  repeated. 

The  varied  forms  of  fissures,  underground  conduits,  and  water  supply 
account  for  the  peculiarities  shown  by  different  geysers.  It  has  been  found 
that  adding  alkaline  substances,  such  as  soap  or  lye,  to  the  waters  of  geysers 
causes  some  of  them  to  erupt  very  quickly;  this  makes  the  water  somewhat 
viscous  and  the  liberation  of  steam  difficult  and  rather  explosive,  leading  to 
sudden  lowering  of  pressure  and  eruption. 

That  the  source  of  heat  for  the  geysers  and  hot-springs  in  the  Yellowstone 
Park  must  be  quite  deeply  seated  is  shown  by  their  occurrence  in,  and  on  the 
shores  of,  Yellowstone  Lake,  an  immense  body  of  very  cold  water,  below 
which  the  rocks  must  be  cooled  to  considerable  depths. 

Hot-spring  Deposits.  —  It  has  been  previously  shown  that  where 
warm  springs,  especially  if  they  contain  carbon  dioxide  in  notable 
quantity,  come  up  through  limestone  beds  they  form  deposits  of  cal- 
careous tufa  or  travertine.  See  page  167.  But  the  waters  of  actively 
boiling  springs  and  geysers,  which  occur  only  in  regions  of  recent 
geological  activity,  and  in  connection  with  lavas,  are  mostly  alka- 
line and  carry  silica,  Si02,  in  solution,  which  they  deposit  as  a 
whitish  material,  varying  from  compact  to  spongy  in  texture,  and 
known  as  geyserite  or,  more  commonly,  siliceous  sinter.  This  forms 
the  geyser  cones,  or  is  'deposited  in  incrustations,  often  of  great 
beauty,  in  and  about  the  margins  of  the  hot-spring  and  geyser 
basins.  The  solutions  are  dilute,  and  the  rate  of  deposition  is  very 
slow  when  it  occurs  only  through  drying,  but  is  hastened  by  the 
action  of  organisms.  Formations  of  rather  considerable  size  and 
thickness  have  been,  and  are  being,  made  in  this  way,  as  seen  form- 
ing the  floor  of  the  basin  in  Fig.  174.  While  hot-springs  and  geysers 
are  not  geological  factors  which  are  of  importance  from  the  magni- 
tude of  the  results  which  they  achieve,  they  are  yet  of  great  signifi- 
cance in  a  proper  understanding  of  certain  processes,  such,  for  in- 


234  TEXT-BOOK   OF  GEOLOGY 

stance,  as  the  deposit  of  certain  ores  of  metals,  and  are  of  wide 
popular  interest. 

It  has  been  found  by  Weed  that,  as  in  the  case  of  travertine,  page  167,  the 
deposit  of  silica  is  very  largely  due  to  the  secretion  of  it  by  low  forms  of 
vegetable  life,  diatoms  and  algae,  the  latter  related  to  sea-weeds,  which  flourish 
in  the  warm  and  even  hot  waters.  The  beauty  of  many  of  the  pools  is  greatly 
enhanced  by  the  rich  coloring  which  these  growths  add  to  them.  It  may  be 
that  they  represent  to  us  some  of  the  earliest  and  most  primitive  types  of 
life  which  existed  on  the  earth. 

Besides,  silica  the  hot-springs  may  form  other  substances ;  the  waters  in  some 
places  are  acid,  and  deposit  sulphur  and  alum  salts.  In  other  cases  sulphides  of 
arsenic  and  of  metals  are  found,  throwing  light  on  the  formation  of  ore  bodies. 


CHAPTER  IX 

MOVEMENTS  OF  THE  EARTH'S  OUTER  SHELL; 
EARTHQUAKES 

Introductory.  —  Experience  and  observation  show  that  the  outer 
shell  of  the  earth  is  not  fixed  and  rigid,  but  undergoes  changes  which 
result  in  movement  of  one  part  of  it  as  compared  with  another. 
The  evidence  is  overwhelming  that  this  has  occurred  repeatedly  in 
the  past,  and  in  all  those  places  where  it  is  permitted  us  to  examine 
the  structure  of  the  earth's  crust.  The  movements  of  the  different 
parts  of  the  outer  shell  have  been  differential  with  respect  to  one 
another,  and  not  only  up  and  down,  but  back  and  forth  in  directions 
tangential  to  the  earth's  circumference.  The  evidence  that  such 
movements  have  taken  place  lies  in  the  results  which  they  have 
achieved,  and  these  we  shall  see  and  study  in  detail  under  the  head- 
ing of  Structural  Geology  in  a  later  part  of  this  work.  Here  it  is 
intended  to  show  that  gradual  and  massive  movements  of  the  crust 
of  the  globe  have  taken  place,  not  only  in  the  remote  but  in  the 
immediate  past,  with  results  of  magnitude,  and  that  they  are  still 
continuing.  We  shall  first  examine  the  evidence  and  then  see  what 
conclusions  may  be  drawn  from  it. 

Datum  Plane.  —  It  is  evident  that  in  order  to  consider  the  rate 
and  extent  of  movement  of  different  areas  of  the  earth's  surface,  or 
even  to  know  that  it  has  occurred,  we  must  have  some  fixed  point  of 
reference.  For  vertical  movements  the  level  of  the  sea  immediately 
suggests  itself  as  a  datum  plane  to  which  they  can  be  referred.  For, 
if  it  can  be  shown  that  relative  displacements  of  land  and  sea  levels 
have  occurred  in  any  place,  it  is  natural  to  think  that  the  movement 
must  be  that  of  the  land,  since  the  sea,  averaging  the  tides,  must 
maintain  a  mean  tidal  level  throughout  its  whole  extent.  Along 
coast-lines,  therefore,  we  use  the  sea  surface  as  the  point  of  refer- 
ence for  vertical  movements  of  the  earth's  outer  shell. 

The  idea  of  the  fixedness  of  the  sea  surface  must  not,  however,  be  carried  too 
far.  As  explained  on  a  previous  page  (90),  the  ocean  does  not  present  us  a 
truly  geometric  surface,  but  a  warped  one.  And,  as  further  explained  under 
coral  islands,  page  188,  the  sea-level  has  varied  within  recent  geologic  times, 
first,  by  the  withdrawal  of  a  part  of  its  water,  due  to  its  accumulating  as  ke 

235 


236  TEXT-BOOK   OF  GEOLOGY 

on  the  land,  and  second,  by  the  restoration  of  this  water  to  the  ocean  basins  by 
melting  of  most  of  the  ice.  And  also  there  are  reasons  for  thinking  that  the 
ocean  has  increased  in  size  and  depth  through  geologic  time  by  the  constant  ad- 
dition of  magmatic  waters,  as  explained  under  volcanoes.  It  is  also  more  than 
probable  that,  as  from  time  to  time  the  earth  shrinks  recurrently,  with  sinking 
and  warping  of  the  floors  of  the  ocean  basins,  and  corresponding  possible  in- 
crease of  velocity  of  rotation,  such  changes  will  cause  movements  of  the  ocean 
waters  on  its  surface.  Such  shif tings  are  registered  by  apparent  up  and  down 
movements  of  the  shores,  giving  rise  to  the  strand-lines  mentioned  below. 
But,  since  such  changes  in  the  ocean  are  very  gradual  and  general,  while  the 
movements  of  the  land  we  are  considering  are  local,  much  more  rapid,  and 
much  greater  in  degree,  we  may  still  for  our  purposes  measure  them  against 
the  sea-level  as  a  relatively  fixed  point. 

Elevation.  —  The  most  striking  proofs  that  upheaval  of  the  land 
from  the  sea  has  occurred  consist  in  the  elevation  of  those  features 


Fig.  181.  —  Ancient  sea-caves  in  former  sea-cliff  at  back  of  elevated  beach,  showing 
strandJine.     Coast  of  Fifeshire,  Scotland.     Geol.  Surv.  of  Scotland. 

which  we  definitely  associate  with  the  sea,  or  its  edge,  so  that  they 
are  now  inland  and  high  above  it.  Thus  in  various  parts  of  the  world 
outcrops  of  rocks  with  dead  marine  organisms,  such  as  barnacles,  or 
other  shells,  and  corals,  still  attached  to  them  are  found  high  above 
sea-level,  or  the  rocks  are  pierced  by  the  tunnels  of  rock-boring 
shelled  animals  (Lithodomus)  which  may  still  contain  their  shells. 
This  shows  that  such  changes  have  recently  occurred,  while  the  pres- 
ence of  the  remains  of  shells  and  other  marine  organisms  as  fossils 


MOVEMENTS  OF   OUTER  SHELL;    EARTHQUAKES         237 

in  the  rocks  of  the  highest  mountain  ranges  proves  that  they  have 
also  happened  in  the  remote  past. 

The  classic  example  of  proof  of  changes  in  land  level  is  found  in  the  temple 
of  Jupiter  Serapis  built  by  the  Romans  near  the  seashore  in  the  vicinity  of 
Naples.  The  three  columns  left  standing  are  bored  by  lithodomi  to  a  height 
of  20  feet  above  the  floor,  and  their  shells  may  yet  be  seen  in  the  holes.  From 
this  we  infer  that  after  the  temple  was  built  the  land  subsided  at  least  20  feet, 
or  more,  carrying  the  temple  into  the  sea,  and  that  since  then  it  has  again 
risen  to  an  equal  amount. 

Another  line  of  evidence  consists  in  the  elevation  of  those  con- 
spicuous features  which  the  edge  of  the  sea  makes  in  its  geologic 


Fig.  182.  —  Elevated  strand-lines  cut  in  sandstones  and  limestones.     Straits  of  Belle 
Isle,  Labrador.     Schuchert  photo. 

work  of  eroding  the  land,  and  which  were  described  in  previous 
pages  (97-105.)  Thus  raised  beaches,  wave-cut  and  wave-built  ter- 
races, forming  level  benches  of  country  terminated  inland  by  sea-cut 
cliffs,  the  latter  often  -pierced  by  wave-formed  caves,  show  the 
elevation  of  a  former  sea-margin,  Fig.  181.  They  are  often  spoken 
of  as  a  strand-line,  and  commonly  appear  as  a  more  or  less  distinct 
topographic  line,  or  level,  approximately  parallel  to  the  present 
shore-line  and  above  it. 

Still  another  line  of  evidence,  of  a  positive  character,  is  found  in 
what  may  be  termed  human  records.  Thus  in  northern  Sweden  the 
uprise  has  been  under  observation  for  a  long  period,  and  has  been 
measured  by  marks  placed  on  the  shore.  In  one  place  the  elevation 
was  about  two  feet  in  a  century,  but  the  rate  is  not  everywhere 


238 


TEXT-BOOK   OF    GEOLOGY 


uniform  and  it  varies  also  from  time  to  time.  All  the  facts  point  to 
the  Scandinavian  peninsula  as  having  gently  risen  for  a  long 
period,  so  that  its  crest  in  the  northern  part  of  Norway  is  1,000 
feet  higher  than  it  once  was.  Raised  stranj^ines  are  a  noticeable 
feature  in  many  northern  regions,  Fig.  182.  Similar  facts  have  been 
recorded  in  other  parts  of  the  world.  Thus  the  raised  strand-lines 
prove  that  within  a  recent  geological  period  the  west  coast  of  South 
America  has  experienced  very  considerable  elevation,  and  the 
process  is  probably  still  going  on. 

Depression.  +-  The  evidence  that  the  land,  in  places,  has  sub- 
sided below  sea-level  is  less  striking  than  that  of  elevation,  but  not 
less  convincing  when  fully  understood.  We  must  here  look  for  a 
different  kind  of  evidence,  for  the  fact  that  the  sea  has  encroached 


Fig.  183.  —  Diagram  showing  submerged  forest,     a,  old  forest  soil  with  stumps 
standing  in  it;  b,  marine  deposits  of  silts  and  sands. 

upon  the  land  is  not  in  itself  a  proof  of  subsidence,  since  this  may 
be  due  to  simple  landward  erosion,  as  previously  explained,  page  101. 
Rather  we  must  seek  for  the  submergence  of  features  which  are 
definitely  characteristic  of  land  surfaces.  Such  are  found,  for  ex- 
ample, in  submerged  forests,  and  in  buildings  or  other  structures  of 
mankind,  now  standing  in  the  water.  Increasing  depth  of  average 
water-level  over  well  known  rocks  or  reefs  in  harbors  is  another 


proof. 


Submerged  forests  are  found  at  various  places  along  the  Atlantic  coast-line 
from  Maine  southward.  The  diagram,  Fig.  183,  shows  the  stumps  still  standing 
in  the  forest  soil,  while  above  are  the  marine  deposits  covering  them  up.  It  is 
clear  that  this  could  occur  only  in  situations,  such  as  protected  nooks  and 
corners  of  estuaries,  sheltered  from  the  waves  of  the  encroaching  sea,  which 
would  otherwise  have  swept  away  the  forest  soil.  Sometimes  the  tree  stumps 
are  now  uncovered  and  may  be  seen  standing  up  on  the  sea-bottom  in  the 
water.  Submerged  peat-bogs  which  have  become  tidal  flats  are  rather  com- 
mon and  tell  the  same  story.  Borings  in  the  deltas  of  great  rivers,  such  as 
the  Mississippi  and  the  Ganges,  as  described  beyond,  show  that  the  subsidence 
may  continue  for  long  periods.  All  the  collected  evidence  goes  to  show  that 
the  Atlantic  seacoast  from  Maine  southward  has  gradually  sunk  within  the 
last  geologic  period;  whether  it  is  still  sinking  is  a  matter  about  which  geolo- 
gists have  not  yet  reached  agreement. 


MOVEMENTS  OF  OUTER  SHELL;    EARTHQUAKES         239 

Drowned  Valleys.  —  The  most  impressive  evidence  of  subsidence 
of  the  land,  when  its  significance  is  understood  and  appreciated,  is 
seen  in  the  irregular  coast-lines  produced  by  the  drowning  of  val- 
leys, with  production  j£  bays  and  estuaries.  This  has  in  part  already 
been  discussed,  page  104,  and  Fig.  80.  The  seaward  extension  of  river 
channels,  such  as  the  Hudson,  across  the  submerged  continental  shelf 
for  long  distances,  points  also  in  the  same  direction,  for  manifestly 
these  great  trenches,  sunk  in  the  sea-floor,  could  not  have  been  cut 
while  the  area  was  covered  with  water,  but  only  by  river  or  glacial 
action,  or  both,  when  it  stood  at  a  higher  level  find  was  a  land 
surface. 

Subsidence  and  Deposit  of  Sediment.  —  It  is  a  commonly  ob- 
served fact  that  in  many  parts  of  the  world,  where  heavy  deposits  of 
sediment  are  being  laid  down  by  rivers  in  the  sea  adjacent  to  the 
coast,  subsidence  of  the  ocean  bottom  is  in  progress.  This  is  noted 
in  the  deltas  of  large  rivers,  like  that  of  the  Mississippi.  Borings 
through  them  show  a  great  depth  of  deposits.  Sometimes  these  are 
marine,  sometimes  fresh-water  in  nature,  as  shown  by  the  shells 
which  they  contain,  alternating  with  beds  of  peat  and  buried  forest- 
growths.  These  facts  show  that  subsidence  has  been  going  on  for 
a  long  period,  and  not  at  an  even  rate,  but  as  an  interrupted  process, 
whose  variations  permitted  land,  fresh-water,  and  marine  deposits 
to  be  formed.  Not  only  is  this  occurring  in  the  present,  but,  as  we 
shall  see  later,  it  has  happened  in  many  places  in  the  past,  so  that 
enormous  thicknesses  of  deposits,  up  to  40,000  feet,  or  even  more, 
have  been  laid  down  in  particular  localities,  which  have  been  after- 
wards raised  and  exposed  to  observation.  Such  great  thicknesses 
of  sediments  are  associated  with  mountains,  as  we  shall  see  later,  for 
elsewhere  they  are  much  thinner.  Since  the  products  of  land  waste 
are  chiefly  deposited  close  to  the  shore,  see  page  106,  and,  as  we  can- 
not imagine  a  depth  of  40,000  feet  at  the  edge  of  the  land,  we  are 
forced  to  believe  that  subsidence  must  have  been  occurring  along 
with  the  deposition,  to  permit  accumulations  of  such  thickness  of 
the  sediments. 

It  has  been  a  view  of  some  geologists  that  the  subsidence  is  caused  by  the 
load  of  accumulated  sediment.  They  consider  the  earth's  crust  to  be  in  such 
a  state  of  equilibrium,  isostatic  balance  so-called,  maintained  by  the  yielding 
of  plastic  material  below,  or  the  rocks  being  forced  under  pressure  to  act  as  if 
plastic,  that  where  the  crust  is  lightened  by  erosion  it  will  rise,  and  where 
it  is  loaded  by  sediment  it  will  sink.  But  it  is  to  be  noted  that  should  eleva- 
tion occur  in  one  area  of  the  crust  and  subsidence -take  place  in  an  adjacent 
one,  erosion  would  tend  to  cut  down  the  rising  area  and  fill  up  the  subsiding 
one.  The  mere  fact  that  shifting  of  material  occurs  is  not  in  itself  a  proof  that 
it  is  the  cause  of  the  change  of  levels;  it  may  be  the  effect  of  such  change 


240 


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rather  than  the  cause.    Probably  it  is  both  cause  and  effect.    The  subject  will 
be  considered  more  fully  in  another  place. 

Evidences  of  Elevation  or  Depression  Inland.  —  We  can  hardly 
assume  that  movements  of  the  shell  involving  changes  of  level  are 
confined  to  the  sea-coasts;  they  must  also  occur  inland,  in  the  in- 
terior of  the  continents.  That  this  has  happened  during  the  past  is 
plainly  shown  by  several  lines  of  evidence:  for  example,  by  the 
bodily  sinking  of  tracts  of  land,  such  as  occurred  at  New  Madrid, 
Missouri,  in  1811-1812,  and  by  the  behavior  of  antecedent  rivers  in 
maintaining  their  courses  through  upwarps  and  thus  forming  can- 


Fig.  184.  —  Showing  tilting  of  lake  basin.  AD,  present  lake  level.  BB,  raised  shore- 
line disappearing  under  lake  at  C.  Land  has  been  depressed  to  south  (S)  and  on 
this  side  the  former  shore-line,  E,  has  been  drowned. 

yons,  as  previously  discussed,  page  76.  But  there  are  also  other 
facts  that  prove  changes  of  level  have  taken  place  very  recently  in- 
land and  are,  perhaps,  still  going  on.  This  is  shown  by  the  tilting  of 
lakes,  as  illustrated  in  the  diagram,  Fig.  184. 

The  Great  Lakes  present  one  of  the  best  examples  of  this.  To  the  north- 
east of  them  the  land  appears  to  have  risen  since  the  retreat  of  the  great  ice 
sheet  in  a  huge  low  dome  or  shield.  Since  they  lie  on  the  southwest  side  of 
this  area  of  doming  they  have  been  tilted  to  the  west  and  south.  On  the  north- 
east side  the  old  strand-lines  are  several  hundred  feet  above  the  present  water- 
level,  and  slope  toward  it  as  they  are  followed  west  and  south.  Since  the 
lakes  discharge  to  the  east,  the  raising  of  their  outlets  has  caused  them  to 
enlarge,  expanding  them  to  the  west  and  south.  The  process,  with  modifica- 
tions, has  continued  down  to  the  present,  with  the  drowning  of  river  mouths 
on  the  south  and  west  sides  of  some  of  the  lakes  (Erie  and  Superior),  and  their 
conversion  into  estuaries,  and  is  probably  still  going  on. 

Warping  movements  downward  may  also  be  shown  by  the  conversion  of  a 
part  of  a  river  valley  into  a  lake.  Thus  downwarping  has  converted  a  portion 
of  the  upper  course  of  the  Ottawa  River  into  a  long  narrow  body  of  water, 
known  as  Lake  Temiskaming.  It  occupies  the  site  of  a  glaciated  valley  and 
is  deep  and  narrow. 

Another  line  of  evidence,  which  so  far  has  not  been  much  studied,  or  well 
established,  consists  in  gradual  changes  in  scenery,  brought  about  by  slow 
warping  effects  of  the  earth's  crust,  like  the  appearance  of  a  distant  object,  such 
as  a  building,  rock,  or  hilltop,  over  the  crest  of  an  intervening  ridge  from 
some  well  determined  spot,  whence  in  previous  years  it  could  not  be  seen.  A 
number  of  such  changes  are  reported,  but  examination  has  thrown  serious 


MOVEMENTS   OF   OUTER  SHELL;    EARTHQUAKES         241 

doubt  upon  their  validity.  Since  they  must  take  place  very  slowly,  and  the 
work  of  erosion  must  also  be  taken  into  account,  until  proper  photographic 
and  surveying  records  have  been  long  established,  no  real  dependence  can  be 
placed  upon  them. 

Classification  of  Movements.  —  In  a  geologic  sense  the  shell  of 
the  earth  is  never  quiet  or  at  rest,  but  it  is  always  undergoing  slow 
motion;  in  one  place  apparently  motionless  for  a  period  but  in 
another  slowly  rising,  in  another  gradually  subsiding.  During  one 
epoch  the  continents  are  heaving  upward  and  the  seas  retreat  from 
their  borders,  at  another  they  are  sinking  and  the  oceans  advance 
and  eat  their  way  inland.  At  tunes  these  motions  have  become 
more  energetic  and  certain  belts  of  the  earth's  shell  have  been 
crushed  together,  both  longitudinally  and  transversely,  with  folding 
and  fracturing  and  the  rising  up  of  mountain  ranges,  as  we  shall 
see  more  in  detail  in  a  later  place.  All  such  movements  of  the  outer 
shell,  whether  of  continental  masses,  or  in  mountain  making,  whether 
of  folding  or  fracturing,  or  dislocation  of  one  part  with  respect  to 
another  part,  whether  upward  or  downward,  or  by  horizontal  thrust- 
ing or  stretching,  are  comprehended  under  the  general  term  of  dia- 
strophism,  and  the  forces  producing  such  results  are  spoken  of  as 
diastrophic.  For  the  sake  of  convenience  also,  when  diastrophic 
forces  affect  and  move  the  continental  masses,  they  are  termed 
epeirogenic,  from  the  Greek  epeiros,  a  continent ;  when  concerned  in 
making  mountain  ranges,  orogenic,  from  the  Greek  oros,  mountain, 
and  gen,  producing. 

These  diastrophic  movements  are  probably  all  to  be  referred  to  the  same 
general  cause,  but  it  is  useful  to  distinguish  between  different  phases  accord- 
ing to  the  results  achieved.  Thus,  in  addition  to  the  terms  epeirogenic  and 
orogenic,  gradual  warping  movements  of  the  land  surfaces,  such  as  those  taking 
place  about  the  Great  Lakes,  have  been  called  bradyseisms  (from  the  Greek 
bradus  and  seismos,  meaning  slow  earthquake). 

Intermediate  between  the  epeirogenic  forces  concerned  in  the  mak- 
ing and  moving  of  continents  and  ocean  basins,  and  the  orogenic 
ones  giving  rise  to  mountain  ranges,  are  those  which  elevate  or  de- 
press great  blocks  of  the  outer  shell.  The  movement  is  essentially 
in  a  vertical  direction.  The  upward  movement  of  such  areas  on 
the  continents  has  given  rise  to  plateaus,  such  as  those  of  the  Colo- 
rado and  of  Thibet,  while  the  downward  one  has  yielded  depressed 
tracts,  like  the  great  Rift  Valley  of  East  Africa  with  its  contained 
lakes.  In  the  ocean  basins  the  submarine  plateaus  and  the  "deeps" 
(see  page  91)  point  to  similar  movements  and  results.  The  areas 
thus  raised  or  depressed  do  not  move  as  units,  but  are  broken  into 
great  blocks  whose  movement  is  attended  with  more  or  less  dis- 
location, the  result  of  which  we  shall  have  occasion  to  consider 


242  TEXT-BOOK   OF   GEOLOGY 

later.  All  the  movements  which  tend  to  produce  changes  in  the 
earth's  surface  are  often  spoken  of  as  deforming,  and  the  results 
achieved  by  them  as  deformations. 

Cause  of  Diastrophism.  —  When  we  endeavor  to  account  for  the 
various  movements  which  the  earth's  outer  shell  has  undergone,  and 
is  still  undergoing,  it  appears  that  we  can  find  for  them  an  immedi- 
ate cause,  but  not  at  present  an  ultimate  one.  Concerning  the  im- 
mediate cause  there  is  quite  general  agreement  of  opinion,  and  it  is 
held  to  be  due  to  the  unequal  contraction  or  shrinking  of  the  earth, 
taken  as  a  whole.  There  is  a  great  body  of  proof  which  shows  that 
the  outer  shell  of  the  earth  has  undergone  this  contraction  and 
that  it  is  probably  continuing;  through  this  the  surface  is  gently 
warped  up  or  down,  and  at  times  there  has  been  more  energetic 
crushing  in  certain  belts  and  areas.  The  proof  of  this  we  shall 
study  in  detail  in  a  later  part  of  this  work. 


Fig.  185.  —  Diagram  of  deforming  movements.  A  and  B,  B  have  sunk  from  the 
original  surface  CDE,  but  B,  B  more  than  A,  so  that  the  latter  appears  to  have 
risen.  A  is  a  horst,  B,  B  are  graben. 

But  concerning  the  ultimate  cause,  the  reason  why  the  earth 
should  contract,  why  it  should  do  so  in  an  irregular  manner,  and 
how  the  shrinkage  causes  diastrophism,  there  are,  indeed,  many 
opinions  but,  as  yet,  unfortunately  not  much  real  knowledge;  what 
is  known  and  the  most  important  views  that  are  held  will  be  dis- 
cussed in  a  later  place ;  before  taking  them  up  we  will  next  consider 
certain  results  of  these  movements,  which  are  not  only  of  interest 
and  importance  in  themselves,  but,  as  it  has  recently  appeared,  have 
taught  us  some  valuable  facts  regarding  the  character  of  the  earth's 
interior,  and  seem  destined  to  teach  more  as  their  study  continues. 
We  refer  to  earthquakes,  whose  consideration  follows  in  the  next 
section. 

In  the  preceding  discussion  of  diastrophism  reference  has  been  made  in  a 
number  of  places  to  upward  and  downward  movements.  These  should  be 
understood  to  be  relative  terms,  the  position  of  one  area  with  respect  to  its 
surroundings.  For  it  is  possible  that  in  the  shrinkage  of  the  earth,  each  of  the 
areas  and  our  datum  plane,  the  ocean  level,  are  all  moving  towards  its  cen- 
tral point,  but  that  some  move  faster,  or  more,  than  others,  and  thus  cause 
these  relative  changes.  See  Fig.  185.  Those  areas  which  stand  at  a  higher 
level,  or  apparently  rise,  are  often  spoken  of  as  positive  elements  of  the  shell, 
in  contrast  to  which  the  depressed  tracts  are  termed  negative  ones,  terms 


MOVEMENTS  OF  OUTER  SHELL;    EARTHQUAKES         243 

whose  significance  will  appear  when  we  study  the  geography  of  the  world  in 
past  geologic  times.  Areas  like  A  in  the  diagram,  Fig.  185,  are  known  as 
horsts,  those  like  B,  B  as  graben,  from  the  German,  the  one  meaning  an  eleva- 
tion, the  other  a  trench,  or  trough. 

The  main  positive  elements  of  the  earth's  crust  are,  or  have  been,  con- 
tinental areas,  the  negative  ones  are  the  deep  ocean  basins.  The  continental 
areas  are  again  broken  into  smaller  tracts,  which  are  subordinate  negative  and 
positive  elements,  according  to  their  motion  with  respect  to  one  another;  it  is 
these  that  are  commonly  distinguished  as  horsts  or  graben. 

Earthquakes 

Introductory.  —  In  the  preceding  section  it  has  been  shown  that 
in  a  geological  sense  the  earth's  shell  is  undergoing  constant  move- 
ment on  a  large  and  massive  scale.  In  what  we  may  term  a  present 
sense  it  is  constantly  subjected  to  relatively  small  and  often  rapid 
motions.  These  may  arise  from  a  great  variety  of  causes,  and, 
ordinarily,  they  are  not  perceived  by  us,  though  they  may  be  de- 
tected by  suitable  instruments.  When  they  occur  as  tremors  which 
we  can  distinctly  recognize  they  are  called  earthquakes.  These  are 
not  only  interesting  in  themselves  as  geological  phenomena,  but  are 
of  such  great  importance  to  humanity,  on  account  of  the  loss  of  life 
and  great  destruction  which  they  frequently  entail,  that  they  have 
been  made  the  subject  of  wide-spread  and  continued  investigation. 
As  a  result  there  is  perhaps  no  field  of  geological  inquiry  in  which 
greater  progress  has  been  made,  especially  in  recent  years,  and  some 
of  the  more  important  facts  and  conclusions  are  here  presented. 
The  study  of  earthquakes  is  known  as  the  science  of  seismology, 
from  the  Greek  seismos,  an  earthquake. 

Cause  of  Earthquakes.  —  An  earthquake  is  a  trembling,  or  un- 
dulatory  motion,  in  the  more  or  less  elastic  rock-shell  of  the  earth, 
communicated  to  it  by  an  impulse  or  shock  of  some  kind,  as  a  bowl 
of  jelly  might  be  set  in  vibration  by  a  smart  tap  on  the  side  of  the 
containing  vessel.  The  shock  or  impulse  is  evidently  the  cause  of 
the  earthquake,  and  the  question  arises,  what  is  the  origin  of  such 
shocks?  The  evidence  shows  that  they  may  arise  from  several 
causes,  most  of  which  must  be  considered  of  minor  importance  com- 
pared with  one  major  source,  which  appears  to  give  rise  to  all  great 
earthquakes. 

One  minor  cause  is  found  in  violent  volcanic  outbursts,  like  that  of  Kraka- 
toa  in  1883  and  of  Bandaisan  in  Japan  in  1888,  but  earthquakes  produced  in 
this  way  are  light  in  intensity  and  quite  limited  in  extent.  Moreover,  many 
outbursts  are  not  attended  by  any  shocks,  or  but  extremely  feeble  ones,  like 
that  of  Mont  Pelee  in  1902.  It  used  to  be  thought  that  volcanic  action  was 
an  important  source  of  earthquakes  and  this  idea  is  frequently  revived;  but 
the  careful  comparison  of  the  two  phenomena,  especially  in  Japan,  has  shown 


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that  there  is  no  necessary  connection  in  occurrence  between  heavy  earthquakes 
and  volcanic  eruptions. 

Another  minor  cause  may  be  found  in  the  sudden  caving  in  of  subterranean 
cavities,  due  to  the  yielding  of  the  roof  to  the  weight  of  superincumbent  rock 
masses.  This  is  most  liable  to  happen  in  limestone  regions,  since  this  rock  is 
apt  to  be  removed  in  solution  by  underground  waters,  as  previously  ex- 
plained, page  162.  It  is  possible,  as  has  been  suggested,  that  the  earthquakes 
which  in  1811  devastated  the  lower  Mississippi  valley,  especially  about  New 
Madrid  in  southern  Missouri,  were  partly  due  to  this  cause,  though  the  area 
affected  is  too  extensive  and  the  effects  of  the  earthquake  shocks  produced 
were  felt  to  too  great  distances  for  it  to  have  been  more  than  a  minor  one. 


Point  Arena 


Fig.  186.  —  Map  of  a  part  of  California,  showing  the  position  and  extent  of  the  fault- 
line,  A-A,  movement  along  which  produced  the  earthquake  of  April  18,  1906. 

It  has  now  been  rather  definitely  settled  that  the  main  cause  of 
earthquakes,  especially  the  heavier  ones,  is  the  jar  given  the  earth's 
shell  by  the  sudden  forming  of  a  fracture  in  its  outer  portion,  or,  per- 
haps, a  sudden  slipping,  or  displacement,  along  the  walls  of  an 
already  existent  fracture.  The  upper  part  of  the  earth's  shell  is 
divided  by  rifts  into  blocks,  both  great  and  small;  such  fractures 
probably  die  out  below  at  a  depth  of  twelve  miles,  or  thereabouts, 
where  the  overlying  weight  exceeds  the  crushing  strength  of  the 
rocks,  though  temporarily,  at  the  time  of  formation,  they  may  pene- 
trate more  deeply.  Above  this  the  blocks  may  adjust  themselves  to 
the  contraction  of  the  earth  as  a  whole  by  settling  down,  and  by 
movement  along  the  walls  of  the  rifts.  Fractures  along  which  dis- 
location has  taken  place  are  called  faults  and,  as  we  shall  see  later, 
such  faults  are  a  matter  of  great  importance  in  structural  geology. 
The  scale  on  which  such  phenomena  take  place  is  very  great;  the 


MOVEMENTS  OF   OUTER  SHELL;    EARTHQUAKES         245 

fault  along  which  abrupt  movement  caused  the  great  earthquake  in 
California  on  April  18,  1906,  has  been  traced,  with  only  two  or  three 


Fig.  187.  —  Trace  of  the  fault  fissure  concerned  in  the  California  earthquake  of  1906. 
G.  K.  Gilbert,  U.  S.  Geol.  Surv. 

interruptions,  a  distance  of  600  miles  and  displacement  occurred 
along  at  least  270  miles  of  it ;  it  is  known  as  the  San  Andreas  Rift. 
See  Figs.  186  and  187.  This  is,  however,  exceptional  and  distances 
of  40-50  miles  are  more  common.  The  fractures,  if  already  existent, 
are  not  necessarily  open,  their  walls  are  pressed  tightly  together,  and 
perhaps  in  places  healed  by  deposited  material.  A  sudden  forced 
motion,  even  of  only  a  few  inches,  along  such  fractures,  with  friction 
and  perhaps  rupture,  where  the  masses  involved  are  so  tremendous, 
is  quite  sufficient  to  generate  a  shock  which  would  produce  a  dis- 
astrous  earthquake.  The  motion  is  commonly  vertical,  and  may 
amount  to  several  feet  and  even  yards,  see  Figs.  187,  188  and  also 
Fig.  277,  but  lateral  and  oblique  movement  is  also  liable  to  occur; 
thus  the  horizontal  displacement  on  the  sides  of  the  fault-line  in 
the  California  earthquake  of  April,  1906,  was  from  8  to  20  feet,  as 
shown  by  the  separated  ends  of  fences,  etc.,  see  Fig.  189,  while  the 


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maximum  vertical  dislocation  was  not  more  than  from  one  to 
three  feet. 

The  most  recent  view  of  the  cause  of  earthquakes,  according  to  H.  F.  Reid, 
is  not  that  the  shock  is  caused  by  the  bodily  slipping  movement  of  a  great 
block  of  the  earth's  shell  in  mass,  but  rather  by  the  sudden  fracture  of  the  rock 
along  a  line  in  an  area  which  has  long  been  under  gradually  accumulating 
strain.  He  states  the  causes  and  effects  as  follows. 

The  fracture  of  the  rock,  which  causes  a  tectonic  earthquake,  is  the  result  of 


Fig.    188.  —  Displacement,  or  fault,   along  a  great  fissure  which  produced  a  heavy 
earthquake  in  Japan  in  1891,  at  Midori  in  the  Neo  Valley.     K.  Ogawa,  photo. 

elastic  strains,  greater  than  the  strength  of  the  rock  can  withstand,  produced  by 
the  relative  displacement  of  neighboring  portions  of  the  earth's  crust. 

These  relative  displacements  are  not  produced  suddenly  at  the  time  of  the 
fracture,  but  attain  their  maximum  amounts  gradually 'during  a  .more  or  less  long 
period  of  time. 

The  only  mass  movements  that  occur  at  the  time  of  the  earthquake  are  the 
sudden  elastic  rebounds  of  the  sides  of  the  fracture  towards  positions  of  no 
elastic  strain;  and  these  movements  extend  to  distances  of  only  a  few  miles  from 
the  fracture. 

The  earthquake  vibrations  originate  in  the  surface  of  fracture;  the  surface 
from  which  they  start  has  at  first  a  very  small  area,  which  may  quickly  become 
very  large,  but  at  a  rate  not  greater  than  the  velocity  of  compressional  elastic 
waves  in  the  rock. 

Effect  of  Shock.  —  The  student  must  carefully  bear  in  mind  the 
difference  between  cause  and  effect  in  earthquake  phenomena; 
thus  in  Fig.  188  the  displacement  shown  is  not  the  result  of  an 


MOVEMENTS  OF  OUTER  SHELL;  EARTHQUAKES  247 

earthquake,  but  the  cause  of  one.  The  effect  of  the  sudden  move- 
ment along  a  fault  line,  or  the  forming  of  a  new  one,  is  that  vibra- 
tions are  sent  outward  in  the  earth  from  that  place,  and  these  are 
the  earthquake,  as  it  is  perceived  at  a  distance.  Within  a  certain 


Fig.  189.  —  Horizontal  displacement,  or  shove,  without  appreciable  vertical  change, 
as  shown  by  the  separated  parts  of  the  fence.  One  is  looking  perpendicularly 
toward  the  plane  of  the  fissure.  California  earthquake  of  1906.  G.  K.  Gilbert, 
U.  S.  Geol.  Surv. 

zone,  on  either  side  of  the  fault  line,  or  on  both  sides,  the  destructive 
effects  observed  in  the  demolition  of  buildings,  etc.,  may  be  chiefly 
due  to  the  sudden  shift  in  the  ground,  especially  if  this  takes  place 
horizontally;  at  increasing  dis- 
tances from  this  line  the  vibra- 
tions are  more  and  more  the 
cause  of  the  different  things 
which  may  happen.  Thus  the 
earthquake  is  propagated  as  a 

Series   Of  Waves    in     the     earth,  Fig>  190.- Wire  model  showing  path  traveled 
as    in    an    elastic    body.       When       by  a  particle  of  matter  during  an   earth- 

these  emerge  at  the  surface  the     <*uake;  after  Sekiva- 

ground  is  thrown  into  very  short,   rapid   vibrations,  which  even 

in  severe  earthquakes  have  a  range  of  only  a  few  inches.  The  waves 


248 


TEXT-BOOK  OF  GEOLOGY 


along  the  surface  move  at  a  rate  of  about  two  miles  per  second. 
The  motion  is  not  only  back  and  forth,  but  also  up  and  down, 
and  the  path  described  by  an  individual  particle  may  be  very 
complicated,  as  illustrated  in  Fig.  190.  The  nature  of  the  elastic 
waves  transmitted  through  the  earth  and  what  they  have  taught 
us  will  be  discussed  in  another  paragraph. 

It  used  to  be  thought  that  earthquakes  were  generated  from  a  point  at  some 
depth,  say  from  two  to  six  miles,  below  the  surface  and  this  was  called  the  focal 
point,  or  centrum.  The  point  immediately  over  this  on  the  surface  was  called 
the  epicenter.  This  latter  point  was  determined  by  drawing  concentric  closed 
curves,  called  coseismal  lines,  on  a  map  of  the  region  through  points  of  simul- 
taneous arrival  of  the  waves,  as  indicated  by  observatories,  clocks,  etc.  See 
Fig.  191.  By  other  mathematical  methods  the  distance  below  the  epicenter  of 
the  focal  point  was  calculated.  These  methods  led  to 
discordant  results  for  a  given  earthquake,  and  even- 
tually to  the  discovery  that  there  might  be  several 
epicenters  situated  in  a  line,  or  that  where  earth- 
quakes habitually  occurred  in  a  given  region  the 
epicenters  were  situated  on  this  line.  Further  inves- 
tigation showed  that  these  were  fault  lines  and  this 
led  to  the  present  understanding  of  their  cause,  as 
previously  stated.  Thus  the  former  ideas  of  a  focal 
point,  of  its  depth  below  the  surface,  etc.,  have  in  large 
measure  lost  their  significance. 

Recent  Examples.  —  On  August  31,  1886,  the  city 
of  Charleston  in  South  Carolina  was  visited  by  a 
severe  earthquake  which  killed  and  wounded  a  number 
of  people  and  did  great  damage  to  buildings  and  other  structures.  The  shock 
was  distinctly  felt  as  far  away  as  Chicago,  a  distance  of  800  miles.  It  has  been 
suggested  that  this  was  caused  by  the  sudden  slipping  seaward  of  vast  masses  of 
sediment  accumulated  on  a  descending  coast-line,  but  the  attendant  phenomena 
leave  little  doubt  that  like  most  other  earthquakes  it  was  due  to  the  settling  and 
adjustment  of  shell  blocks. 

On  May  3,  1887,  a  tremendous  earthquake  occurred  in  the  province  of  Sonora 
in  northern  Mexico.  It  was  felt  over  the  greater  part  of  New  Mexico  and  Arizona, 
but  as  these  were  then  very  thinly  settled  regions  little  damage  was  done.  The 
fault  occurred  at  the  base  of  a  mountain  range  which  was  uplifted  in  places 
twenty  feet. 

In  1899,  a  great  earthquake  took  place  in  southern  Alaska.  As  the  region  is 
mostly  uninhabited  it  passed  almost  without  notice  at  the  time.  Studies  which 
have  since  been  made  show  that  considerable  alterations  in  topography  took 
place  at  the  time  of  its  occurrence,  especially  about  Yakutat  Bay.  Marked 
changes  were  also  induced  in  the  great  glaciers  of  this  region  (page  128)  by  the 
shattering  of  the  ice  and  by  snow  slides  from  the  mountains.  See  Fig.  277. 

On  April  18,  1906,  occurred  the  great  earthquake  in  central  California  which 
has  been  previously  mentioned.  The  loss  of  life,  from  various  causes  may  have 
reached  1,000;  many  towns  and  cities  were  greatly  damaged;  but  the  chief  de- 
struction took  place  in  San  Francisco.  The  city,  damaged  by  the  shock,  was  in 


Fig.    191.  — Map    of 
coseismic  lines. 


MOVEMENTS  OF  OUTER  SHELL;    EARTHQUAKES         249 

great  part  destroyed  by  a  resulting  conflagration,  which  could  not  be  checked 
because  the  pipes  carrying  the  water  supply  were  laid  across  the  fault-line,  and 
the  displacement  cut  them  in  two. 


Fig.  192.  —  Destruction  caused  by  earthquake  vibrations,  Stanford  University,  Cal., 
April,  1906.     W.  C.  Mendenhall,  U.  S.  Geol.  Surv. 

In  August,  1906,  the  coast  of  Chile  was  visited  by  a  severe  earthquake,  which 
did  great  damage  in  Valparaiso  and  other  places.  The  number  of  persons 
killed  was  estimated  at  several  thousand.  After-shocks  continued  for  a  long 
time  while  readjustment  along  the  fault  surface  was  going  on.  The  west  coast 
of  South  America  is  noted  for  its  earthquakes,  in  connection  with  which  not- 
able elevation  of  the  coast-line  has  occurred. 

On  January  14,  1907,  a  heavy  earthquake  happened  at  Kingston,  Jamaica, 
with  destruction  of  property  and  changes  in  the  coast-line  and  in  the  harbor, 
due  to  faulting. 

The  greatest  disaster  of  this  kind  in  modern  times  occurred  on  Dec.  28,  1908, 
when  Messina  and  Reggio,  cities  on  the  narrow  strait  which  separates  Sicily 
from  the  mainland  of  Italy,  were  completely  destroyed  by  a  terrific  shock.  It 
is  estimated  that  possibly  200,000  lives  were  lost  in  this  frightful  catastrophe. 
The  region  has  repeatedly  suffered  from  this  cause  in  previous  times;  it  is 
one  in  which  crustal  readjustment  is  constantly  going  on. 

These  are  only  a  few  examples  out  of  many  that  might  be  selected.  Scarcely 
a  day  passes  that  shocks  are  not  recorded  from  some  part  of  the  world  by  the 
earthquake  observatories. 

Seismic  Belts.  —  Observation  shows  that  although  earthquakes 
occur  in  all  parts  of  the  world  they  are  most  likely  to  happen  in 


250 


TEXT-BOOK  OF  GEOLOGY 


Fig.  193.  —  Map  of  seismic  belts  in  the  Eastern  Hemisphere, 
stereographic  projection. 


On  S.  L.  Penfield's 


certain  well-defined  tracts,  which  lie  in  what  we  may  call  the  great 
seismic  belts.  These  surround  the  earth  as  zones  roughly  in  the 
direction  of  great  circles  which  cross  at  an  angle  of  nearly  70  degrees. 
One  belt  follows  the  western  coast  of  North  and  South  America, 
the  Aleutian  Islands  and  the  island  groups  along  the  eastern  coast 
of  Asia  and  thus  defines  the  Pacific  Ocean.  The  other  includes  the 
Mediterranean,  the  Alps,  the  Caucausus,  the  Himalayas  and  so  on 
into  the  East  Indies.  These  are  shown  on  the  accompanying  maps 
in  Figs.  193  and  194.  It  will  be  noticed  that  in  a  general  way  they 
coincide  with  the  great  volcanic  belts  previously  described,  page  204, 
and  thus  tend  to  show  that  there  is  a  common  cause  for  both  sets 
of  phenomena.  It  is  also  a  notable  fact  that  where  these  belts  lie 
along  the  continental  coasts,  as  in  North  and  South  America  and 
on  the  eastern  coast  of  Japan,  the  land  descends  very  sharply,  with- 


MOVEMENTS  OF  OUTER  SHELL;  EARTHQUAKES  251 


Fig.  194.  —  Map  of  seismic  belts  in  the  Western  Hemisphere. 

stereographic  projection. 


On  S.  L.  Penfield's 


out  any  broad  intervening  shelf,  to  the  depth  of  the  ocean.  See  page 
83.  These  slopes  are  the  edges  of  concave  tracts  of  the  ocean 
floor,  from  100  to  300  miles  wide,  which  border  the  continents 
and  which  appear  to  be  sinking;  while,  conversely,  the  bordering  land 
areas  appear  to  be  rising,  and  in  long  stretches  have  been  elevated 
as  mountain  chains,  as  we  shall  see  later.  These  are  the  belts  or 
zones  of  weakness  in  the  earth's  crust  where  the  stresses  and  strains 
incidental  to  the  contraction  of  the  earth  are  being  constantly  re- 
lieved by  movements,  and  in  which,  therefore,  earthquakes  are  con- 
tinually recurring. 

It  is  often  thought  that  certain  regions  are  practically  exempt  from  danger  of 
earthquakes  because  no  real  disaster  has  happened  in  them  since  they  have 
been  settled  and  cities  have  sprung  up.  Yet  in  New  England,  for  example,  in 
the  230  years  following  its  settlement  over  230  distinct  earthquakes  have  been 
recorded,  an  average  of  one  a  year,  though  probably  none  have  been  of  the  first 


252 


TEXT-BOOK  OF  GEOLOGY 


magnitude.  Where  the  shocks  are  frequently  recurrent  and  slight,  the  danger 
of  a  large  movement  and  heavy  shock  seems  less;  where  quiet  has  long  reigned 
in  a  seismic  belt,  the  shock  which  eventually  comes  is  apt  to  be  severe,  suggest- 
ing that  the  strain  in  the  one  case  is  constantly  eased,  in  the  other  cumulative. 
It  has  also  been  noticed  that  a  heavy  shock  in  one  seismic  belt  seems  to  be 
followed,  not  long  after,  by  one  in  a  very  distant  belt,  rather  than  by  one  in 
neighboring  regions,  as  if,  locally,  the  stresses  and  strains  were  eased.  This  is 
illustrated  by  the  Valparaiso  earthquake  which  followed  soon  after  the  San 
Francisco  one  in  1906. 

Submarine  Earthquakes ;  Tsunamis.  —  What  is  stated  in  the 
foregoing  discussion  of  seismic  belts  suggests  that  a  large  part,  pos- 
sibly the  greater  part,  of  earthquakes  takes  place  on  the  ocean  bot- 
tom, on  the  descending  sides  of  the  deeps.  That  they  do  occur 
beneath  the  sea  is  shown  in  several  ways,  such  as  the  shocks  com- 
municated to  vessels  on  the  surface  above,  and  by  the  rupturing  of 
submarine  cables.  And  with  the  sensitive  instruments  by  which,  as 
will  be  shown  later,  it  is  now  possible  to  record  distant  earthquakes, 


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12  A.M.  2  A.M.        4  A.M.         6  A.M.         8  A.M.       10  A.M.       12  Noon       2  P.M.         4  P.M.         6  P.M.         8  P.M.        10  P.M.  12  A.M. 

Fig.  195.  —  Record  of  tsunami  by  tidal  gauge.  Vertical  lines  represent  time  spacing 
on  the  paper,  driven  horizontally  by  clockwork.  Horizontal  lines  show  height  in 
feet  as  recorded  by  the  rising  and  falling  pencil  of  the  gauge. 

and  determine  their  place  of  occurrence,  many  are  found  to  happen 
beneath  the  sea.  The  most  important  thing  which  these  submarine 
earthquakes  cause  is  the  huge  wave  which  they  may  generate  in  the 
sea.  Such  waves  have  long  been  known  under  the  title  of  tidal 
waves,  a  misleading  name  since  they  have  no  connection  with  the 
tide;  they  are  now  generally  called  tsunamis,  from  their  Japanese 
name,  by  seismologists.  They  may  be  of  immense  size,  from  100  to 
200  miles  from  crest  to  crest,  and  at  the  point  of  origin  40  feet  high. 
They  are  so  broad  that  in  the  open  sea,  unlike  wind  waves  (see 
page  88)  they  would  not  be  perceived;  but  if,  on  approaching  the 
coast,  they  are  still  of  considerable  height  they  may  pile  up  in  huge 
breakers  and,  sweeping  far  inland,  cause  enormous  damage  and  loss 
of  life. 


Lisbon  in  1755,  Japan  in  1854  and  in  1896,  Peru  in  1868,  suffered  well-known 
instances  of  great  and  disastrous  tsunamis,  the  number  of  victims  in  some  cases 


MOVEMENTS  OF  OUTER  SHELL;  EARTHQUAKES  253 


being  20,000.  These  vast  waves  are  felt  over  whole  oceans  and  move  with  tre- 
mendous speed,  from  300  to  500  miles  per  hour.  Those  from  Japan  have  crossed 
the  Pacific  in  nearly  12  hours,  and  are  registered  by  tidal  gauges.  At  such  dis- 
tances their  height  may  be  only  a  few  inches;  but  the  ebb  and  flow  of  from  15  to 
30  minutes,  like  small  subordinate  tides  on  the  top  of  the  regular  tide,  would  be 
registered  as  wavy  lines  by  the  instrument.  See  Fig.  195.  It  is  these  records 
which  enable  us  to  determine  the  size  of  the  wave  since  they  give  the  time  of 
oscillation;  the  speed  between  distant  points  being  known,  the  size  =  velocity  X 
time  of  oscillation. 

It  has  been  supposed  that  these  waves  were  due  to  the  uplifting  or  depressing 
of  an  area  of  the  sea  bottom  by  the  shock,  but  Reid  has  recently  shown  that  it  is 
much  more  probable  that  they  are  caused  by  the  elastic  rebound  of  the  crust 
which  follows  when  it  has  been  gradually  strained  to  such  an  extent  that  a  line 
of  fracture  finally  and  suddenly  occurs,  with  quick  movement  on  either  side  of 
the  break,  as  in  the  California  earthquake  of  1906. 

Recording  Earthquakes ;  Seismographs.  —  Very  delicate  instru- 
ments have  been  invented,  called  seismographs,  which  record  the 
tremors  due  to  distant  earthquakes, 
and  the  study  of  these  records  has 
led  to  important  geological  conclu- 
sions. The  principle  upon  which  such 
instruments  are  now  constructed  is 
simple;  if  a  heavy  mass  of  metal  be 
suspended  like  a  pendulum,  owing  to 
its  inertia  it  will  remain  for*  a  long 
time  at  rest  when  the  shock  arrives, 
while  the  earth  vibrates  beneath  it. 
A  point  or  pencil  of  some  kind  is 
secured  to  the  suspended  weight, 
while  under  it,  on  a  bed-plate  rig- 
idly attached  to  the  earth,  is  a  paper 
or  other  medium  suitably  prepared 
to  record  the  motions  of  the  point 
which  lightly  touches  it.  If  the 
earth  oscillates  beneath  the  sus- 
pended pencil  a  Series  of  lines  Will  Fig.  196.  —  Seismograph  with  astatic 

be  drawn  on  the  vibrating  paper.  If  pendulum,  Wiechert's  model, 
the  paper,  instead  of  being  made  fast,  be  a  strip  continuously 
carried  along  by  clockwork  the  pencil  when  at  rest  will  draw  a 
straight  line  upon  it;  when  vibrations  of  the  earth  occur  the  line 
will  be  broken  and  will  oscillate  sinuously  from  one  side  to  the  other. 
See  Fig.  197.  Such  a  record  is  known  as  a  seismogram. 

While  in  principle,  a  modern  seismograph  is  simple,  in  construction  H  is  a 
rather  complicated  instrument  (see  Fig.  196)  since  it  is  arranged  to  record  not 


254 


TEXT-BOOK   OF  GEOLOGY 


only  horizontal  motion  in  two  directions,  East- West  and  North-South,  but  also 
the  vertical  motion  as  well.  It  is  from  such  records  in  three  directions  that  the 
wire  models  shown  on  page  247  are  constructed.  Since  the  intervals  of  time  are 
marked  on  the  moving  paper,  the  instrument  records  the  time  of  arrival  of  the 
shock  and  also  the  duration.  The  directions  of  diversion  of  the  markers  from 
their  regular  paths  show  also  the  direction  from  which  the  shock  has  come. 

Seismograms. — The  study  of  seismograms  of  distant  earth- 
quakes has  led  to  the  discovery  that  the  main  shock  is  preceded  by 
smaller  quick  vibrations  which  are  recorded  when  the  seat  of  dis- 
turbance is  greater  than  1,000  kilometers  (about  620  miles)  from 
the  recording  station.  These  are  known  as  the  preliminary  tremors. 


Fig.  197.  —  Record  of  the  earthquake  in  Messina  on  Dec.  28,  1908,  as  shown  by  a 
seismograph  in  Gottingen,  Germany,  over  1000  miles  distant. 

Thus  a  normal  seismogram  has  the  characters  seen  in  Fig.  198. 
From  a  number  of  considerations  it  is  now  agreed  that  these  pre- 
liminary tremors  represent  the  shock  which  has  come  by  the  quick- 
est path  through  the  earth,  that  is,  in  the  general  direction  of  a  chord 
from  the  seat  of  disturbance  to  the  recording  station,  whereas  the 
large  vibrations  (see  Fig.  198)  represent  those  waves  that  have  trav- 
eled by  a  longer  route  over  the  surface  circumference.  The  time  be- 
tween the  arrival  of  the  preliminary  tremors  and  that  of  the  main 
shock  is  proportional  to  the  distance  of  the  place  of  disturbance,  and 
the  following  rule  has  been  worked  out  which  gives  roughly  the  dis- 
tance from  the  seat  of  shock:  the  duration  of  the  first  preliminary 
tremors  in  minutes  (and  fractions  of  minutes) ,  less  one,  is  the  dis- 
tance of  the  place  of  disturbance  in  thousands  of  kilometers  (1,000 
kilometers  =  621  miles  approximately) . 


MOVEMENTS   OF  OUTER  SHELL;    EARTHQUAKES         255 


It  is  now  generally  agreed  that  the  first  preliminary  tremors,  coming  through 
the  earth,  are  longitudinal  waves  of  compression,  the  direction  of  vibration  of  a 
point  being  in  that  of  the  line  of  propagation,  that  is,  to  and  fro,  whereas  in  the 
second  preliminary  tremors  coming  through  the  earth  the  wave  is  one  of  dis- 
tortion, with  the  directions  of  vibration  transverse,  in  a  plane  normal  to  the 
path  traveled.  On  the  surface  these  waves  are  "rocking"  ones. 

The  large  surface  waves  passing  around  the  world  in  the  direction  opposite  to 
that  between  the  points  of  the  seat  of  shock  and  the  recording  station,  and  thus 
through  their  antipodes,  have  also  been  detected  by  the  seismograph. 

If  the  distances  from  three  recording  stations  are  known,  then  by  drawing 


Fig.    198.  —  Seismogram  of   distant  earthquake;   ab,   first  preliminary  tremors;    be, 
second  preliminary  tremors;  ce,  main  shock;  fh,  later  phases;  hi,  tail  (after  Omori). 

circles  on  the  globe  with  these  distances  as  radii  and  the  stations  as  centers,  the 
place  of  disturbance  may  be  determined  by  the  intersection  of  the  three  circles. 
In  this  way  the  place  of  shock  has  been  located  in  some  instances  on  the  ocean 
floor. 

Geological  Deductions  from  Seismograms.  —  The  fact  that  the 
preliminary  tremors,  which  are  supposed  to  travel  through  the 
earth,  arrive  at  distant  points  so  long  a  time  ahead  of  the  main 
shock,  cannot  be  explained  alone  by  the  shorter  path  traveled. 
The  time  interval  shows  that  they  are 
also  propagated  at  a  much  greater 
rate  of  speed  than  the  vibrations  trav- 
eling in  the  outer  shell  of  the  earth. 
The  deduction  from  this  is  that  they 
move  in  a  denser,  more  elastic  medium 
which  enables  them  to  gain  speed  as 
they  go.  Moreover,  the  concordant 
results  in  different  directions  show  that 
inside  of  the  outermost  layer,  which  we 
know  is  heterogeneous  in  composition,  Fig.  199.  _  Paths  of  transmission 
the  earth  is  homogeneous,  or  regularly  Of  earthquake  shock  through 
arranged  around  its  center  in  structure,  the  earth-  s> seat  of  shock;  *• 

.  f  ,  .-,         i     ,  y.  z.  recording  stations. 

or,    if    non-homogeneous,    the    hetero- 
geneous parts  are  relatively  so  small  and  numerous  that  different 
paths   of   considerable   length   through   them   give   the   effect   of 
uniformity.     Moreover,  the   average  velocity   increases  with  the 
distance  of  the  recording  station,  thus  the  rate  of  transmission  along 


256 


TEXT-BOOK  OF  GEOLOGY 


sz,  Fig.  199,  is  greater  than  along  sy,  which  in  turn  is  greater  than 
along  sx.  Reid  has  calculated  from  the  data  afforded  by  the  Cali- 
fornia earthquake  that,  at  the  depths  below  the  surface  given,  the 
velocity  of  the  first  preliminary  tremors  is  as  seen  below : 


Depth  below  sur- 
face, miles 

Speed  in  miles 
per  second 

0 

4.5 

272 

6.0 

612 

6.9 

1,225 

7.8 

1,968 

7.9 

These  results  show  not  only  that  velocity  increases  with  the  depth, 
but  more  and  more  slowly  as  the  depth  increases,  and  this  would 
seem  to  indicate  that  the  density  and  elasticity  of  the  earth  increases 
with  the  depth  down  to  a  certain  region.  Seismographs  of  sufficient 
accuracy  and  delicacy  have  not  yet  been  so  generally  installed  that 
data  concerning  earthquakes  at  distances  as  great,  or  greater,  than 
one-third  of  the  earth's  circumference  are  sufficiently  reliable  for  us  to 
draw  very  definite  conclusions  from  them.  The  chord  connecting 
the  ends  of  the  arc  of  one-third  the  earth's  circumference  (120°)  cuts 
the  earth's  radius  at  its  middle  point,  and  thus  it  is  this  outer  shell, 
2,000  miles  thick,  concerning  whose  density  and  elasticity  the  seismo- 
graphs have  so  far  given  us  the  most  information. 

From  the  fact  that  the  rate  of  speed  increases  with  the  depth,  in 
the  outer  shell,  2,000  miles  thick,  it  follows  that  the  quickest  path 
of  wave  transmission  from  the  seat  of  shock  to  a  distant  station  in 
this  portion  of  the  globe  will  not  be  a  straight  line,  as  from  s  to  y 
in  Fig.  199,  along  the  chord  of  the  arc,  but  will  be  a  curved  line 
concave  upward,  somewhat  as  the  line  scy.  In  other  words,  by 
following  this  line  the  waves  gain  more  in  time  in  entering  more 
elastic  layers  than  they  lose  in  distance,  hence  seismologists  gener- 
ally assume  that  the  path  followed  by  the  waves  making  the  first 
preliminary  tremors  at  a  distant  recording  station  is  a  curved  one. 
This  is  of  some  importance  because,  assuming  the  path  to  be  a 
straight  one  and  noting  the  fact  that  the  preliminary  tremors  do  not 
generally  show  in  seismograms  unless  the  distance  is  greater  than 
1,000  kilometers,  the  deduction  has  been  drawn  that  there  must  be 
a  sharp  boundary  between  the  outer  rocky  heterogeneous  shell  of 
the  earth  and  the  inner  homogeneous  core,  and  that,  since  the  chord 
of  an  arc  of  1,000  kilometers  at  its  middle  point  is  12  1/2  miles  below 


MOVEMENTS  OF  OUTER   SHELL;   EARTHQUAKES          257 

the  surface,  this  must  be  the  thickness  of  the  outer  layer.  Others 
assuming  a  curved  path  have  made  the  thickness  as  much  as  800 
miles.  But  Reid  has  shown  that  the  probable  reason  why  the  pre- 
liminary tremors  do  not  show  in  the  records  of  'near'  earthquakes 
is  that  instruments  are  not  generally  sufficiently  delicate  to  record 
and  distinguish  them  as  distinct  from  the  principal  shock,  until 
distance  produces  time  intervals  great  enough  to  be  recognized. 

We  have  not  yet  knowledge  enough  of  the  earth's  interior,  nor  are 
the  data  yielded  by  the  seismographs  from  earthquake  shocks, 
though  promising,  sufficiently  accurate  and  comprehensive  for  us 
to  fix  the  limits  of  the  outer  shell  of  the  earth,  if,  indeed,  there  can 
be  said  to  be  a  very  definite  one.  This  subject  will  be  further  con- 
sidered in  a  later  chapter. 

Geological  Effects  of  Earthquakes.  —  There  are  several  geological 
effects  from  earthquakes,  but  they  are,  comparatively  speaking,  of 
minor  importance.  The  earth  is  often  ruptured  by  the  passage  of 
the  wave  with  the  formation  of  fissures,  which  may  be  of  some  depth. 
A  more  important  one  is  the  starting  of  landslides  and  avalanches  in 
mountainous  regions,  through  the  jarring  of  the  earth.  A  variation 
in  the  flow  of  water  from  springs,  or  the  forming  of  new  ones,  has 
also  been  observed. 

Much  more  important  are  the  movements  of  the  shell  blocks 
which  cause  earthquakes,  but  as  previously  remarked,  these  are  the 
cause  and  not  the  effect  of  the  shocks;  these  movements  have  been 
considered  and  the  results  which  they  produce  are  treated  under 
Structural  Geology. 


DIVISION   II.     STRUCTURAL  GEOLOGY 

CHAPTER  X 
GENERAL  STRUCTURE  AND  PROPERTIES  OF  THE  EARTH 

In  the  foregoing  pages  there  has  been  given  a  general  description 
of  the  various  processes  which  have  been,  and  still  are,  modifying  the 
outer  portion  of  the  earth,  the  part  which  is  directly  open  to  in- 
vestigation and  study.  In  considering  these  agencies  we  have,  to 
some  extent,  been  led  to  notice  the  material  upon  which  they  operate, 
and  the  results  which  they  have  achieved,  but  this  has  been  done  only 
in  so  far  as  was  necessary  to  understand  the  principles  involved,  and 
therefore,  only  in  a  superficial  way.  It  is  now  appropriate  that  these 
materials  and  results  should  be  more  fully  treated,  and  we  shall 
commence  by  considering  the  earth  as  a  whole,  in  relation  to  its 
general  structure,  and  the  properties  it  is  known  to  possess,  and  in 
stating  briefly  the  ideas  which  are  held  regarding  the  nature  of  its 
interior. 

The  Earth  and  Its  Neighbors.  —  The  earth  is  one  of  a  group  of 
planets  which  revolve  around  a  common  central  orb  —  the  Sun. 
Some  of  these,  like  Jupiter,  are  ^ — 
much  larger  than  the  earth,  some  .^^^ajth ^^^^ 

like  the  asteroids,  or  minor  plan-    f     f  /  x-^  \ 

,  (  (   )Sun          ) 

ets,  are  much  smaller;  some  are     V     V    V  ^-^  J 

much  nearer  the  sun,  others  far-      ^ _^™**'^ 
ther  away.     The  group  has  very 

nearly  a  Common  plane  of  revolu-  Fig.  200.  —  Planets  revolve  about  the  sun 
tion  about  the  SUn,  as  Suggested  in  nearly  one  Plane  as  suggested  by  three 

in  Fig.  200,  and  this  fact  is  held      of  them< 

to  have  an  important  bearing  on  the  origin  of  the  system.  The 
path  of  the  earth  about  the  sun  is  not  a  circle,  but  an  ellipse,  one 
of  whose  foci  is  the  sun;  the  deviation  of  the  ellipse  from  a  circle 
is  relatively  small;  the  average  distance  of  the  earth  from  the  sun  is 
nearly  93  millions  of  miles,  it  is  about  \\  millions  of  miles  nearer, 
or  farther,  from  the  sun  according  to  its  place  in  its  elliptical  path. 
The  eccentricity  of  the  ellipse  varies  during  long  periods  of  time, 
from  100,000  to  200,000  years,  and  at  a  maximum  the  earth  may  be 
over  13,000,000  of  miles  nearer  the  sun  in  summer  than  in  winter. 


STRUCTURE  AND  PROPERTIES  OF  THE  EARTH    259 

As  we  shall  see  later,  this  has  been  held  by  some  to  be  a  cause  suffi- 
cient to  produce  great  climatic  changes  and  glacial  epochs.  Besides 
revolving  about  the  sun,  the  earth,  as  is  well  known,  is  spinning 
on  its  own  polar  axis,  [each  revolution  in  24  hours  giving  rise  to 
day  and  night.  This  axis  is  not  perpendicular  to  the  plane  of  the 
earth's  orbit,  but  inclined  to  it  at  an  angle  of  about  23J°,  and  this 
gives  rise  to  the  seasons,  summer  and  winter,  alternately  in  each 
hemisphere. 

The  earth  is  a  very  insignificant  fraction  of  the  universe  as  the  latter  is  known 
to  us,  and  for  that  matter  so  is  the  solar  system  itself.  Nevertheless,  throughout 
the  vast  extent  of  the  universe,  with  its  myriads  of  suns  and  solar  systems  in 
various  stages  of  development,  the  same  general  physical  laws,  which  we  know 
upon  the  earth,  appear  to  govern.  Gravity  operates  in  the  same  manner;  light 
is  transmitted  everywhere  by  the  same  kind  of  vibrations;  the  spectroscope  tells 
us  that  the  same  chemical  elements  are  found  in  distant  suns  and  meteors  as  on 
our  earth.  Nor  do  the  meteorites  which  we  gather  in  our  journey  through  space, 
and  which  appear  like  the  disrupted  fragments  of  former  worlds,  bring  to  us  sub- 
stances strange  to  the  earth.  There  appears,  consequently,  to  be  a  unity  of  law 
and  a  uniformity  of  material  throughout  space,  and  we  consequently  feel  justified 
in  assuming  that  facts  and  reasoning  derived  by  astronomical  study  of  the  other 
heavenly  bodies  may  be  logically  applied  in  our  study  of  the  earth. 

Form  of  the  Earth.  —  The  earth  is  not  a  true  sphere,  but  a  sphe- 
roid, flattened  at  the  poles,  and  the  polar  axis,  or  diameter,  on  which 
it  revolves,  is  about  26  miles  less  than  an  equatorial  one.  This 
oblateness,  or  bulging  at  the  equator,  is  the  form  naturally  assumed 
by  a  revolving  mass,  which,  like  a  liquid,  is  free  to  assume  its  shape 
in  response  to  the  forces  acting  upon  it;  a  mass  of  liquid  in  space, 
free  from  outside  forces,  would  become  a  sphere  through  the  power 
of  its  own  gravitative  attraction;  if  revolved  it  would  bulge  at  the 
equator  and  flatten  at  the  poles,  and  the  amount  of  distortion  would 
depend  on  the  speed  of  rotation.  It  is  held  that  the  degree  of  dis- 
tortion of  the  earth  stands  in  direct  relation  to  its  mass  and  rate  of 
rotation. 

It  is  argued  by  some  that  this  is  a  proof  that  the  earth  was  once  in  a  liquid  con- 
dition, but  this  is  not  a  necessary  conclusion.  For  if  the  forces  tending  to  distort 
the  earth  were  greater  than  the  rigidity  it  possesses,  the  earth  would  yield  to  them, 
no  matter  whether  it  were  liquid,  or  solid  throughout.  Moreover,  the  ideas  of 
liquid  and  solid,  and  the  notions  of  rigidity  which  we  attach  to  them,  when  re- 
ferred to  the  vast  bulk  of  the  earth,  and  the  enormous  forces  of  several  kinds 
which  govern  its  condition,  have  relatively  little  meaning. 

It  has  been  suggested  that  the  rate  of  rotation  has  been  gradually  lessening, 
or  in  other  words  the  day  has  been  growing  longer,  during  geologic  time.  The 
reason  advanced  for  this  is  that  the  tides,  sweeping  across  the  oceans  in  a  direc- 
tion opposite  to  that  of  rotation,  on  being  checked  by  striking  against  the  conti- 
nents, act  as  a  brake  which  tends  to  retard  the  rapidity  of  revolution.  If  this  be 


260  TEXT-BOOK  OF  GEOLOGY 

admitted,  and  also  the  view  that  the  form  of  the  earth  is  in  relation  to  the 
of  rotation  as  mentioned  above,  it  would  follow  that  its  surface  area  has  been 
decreasing  throughout  geologic  time.  For,  if  the  speed  of  rotation  should 
lessen,  the  amount  of  oblateness  would  also  decrease,  and  the  earth  approach  a 
more  spherical  shape.  But  since  the  sphere  is  that  form  in  which  a  given  mass 
of  matter  has  the  smallest  surface  area,  the  latter  would  also  decrease.  But 
in  regard  to  this  view  it  has  recently  been  demonstrated  by  Chamberlin  and 
others,  after  careful  computations,  that  the  amount  of  retarding  effect  of  the 
tides  on  the  earth's  rotation  is  so  small  that  it  must  be  considered  as  a  neg- 
ligible factor.  The  geologic  evidence  is  also  decidedly  against  the  view  that 
during  geologic  time  there  has  been  any  definite  change  in  the  form  of  the 
earth  through  decrease  in  its  oblateness!  The  evidence  will  be  considered 
later  under  mountain  ranges. 

Others  hold,  however,  that  the  earth  has  been  contracting,  and,  since  a  con- 
tracting body  tends  to  revolve  faster,  this  should  counteract  any  retarding 
effect  of  the  tide.  Thie  effect  when  studied  geologically  also  seems  negligible, 
in  that  the  amount  of  shrinkage  during  the  period  recorded  by  geological 
events  has  been  too  small  to  cause  such  a  change  in  the  rate  of  rotation  as 
should  affect  the  form  of  the  earth. 

Density  and  Rigidity  of  the  Earth.  —  The  density,  or  specific 
gravity  of  the  earth,  as  determined  in  several  ways,  is  about  5.6. 
The  density  of  the  outer  shell  is  about  2.7,  and  this  indicates  that  the 
interior  must  be  different  in  constitution  from  the  outer  part.  If 
the  outer  shell  is  relatively  thin,  the  density  of  the  inner  core  need 
not  be  very  different  from  that  of  the  earth  as  a  whole.  If,  on  the 
other  hand,  the  outer  portion  of  the  earth  be  considered  rather  thick, 
a  considerable  fraction  of  the  earth's  radius,  then  the  density  of  the 
interior  must  be  proportionately  higher.  The  indications  given  by 
the  study  of  distant  earthquakes  seem  to  favor  the  view  that  there 
is  no  definite  crust,  and  that  the  density  increases  gradually  with  the 
depth. 

The  rigidity  of  the  earth,  or  its  capacity  for  resisting  deformation, 
and  its  elasticity,  by  virtue  of  which  it  tends  to  resume  its  original 
shape  when  deformed,  are  relatively  high,  as  much  as  one  and  a  half 
times  that  of  hard  steel,  and  perhaps  more. 

The  rigidity  of  the  earth  is  shown  by  its  capacity  to  resist  the  deforming  ten- 
dency produced  by  the  attractive  forces  of  the  sun  and  moon.  We  see  the 
effect  of  these  on  the  watery  envelope  of  the  earth  in  the  production  of  tides, 
and  the  fact  that  the  earth  retains  its  shape  in  spite  of  them,  is  the  strongest 
proof  we  have  that  its  interior  is  not  in  a  liquid  condition,  in  the  sense  in  which 
we  use  that  term  on  its  surface,  as  was  once  firmly  believed.  For,  if  the  earth, 
or  any  considerable  portion  of  it,  were  a  liquid  covered  by  a  relatively  thin 
shell,  we  should  have  interior  tides  and  consequent  displacements  of  the  outer 
crust,  which  is  not  the  case.  Thus,  whatever  may  be  the  condition  of  the  in- 
terior, it  possesses  that  degree  of  rigidity  which  we  associate  with  solid  bodies. 

Its  highly  elastic  nature  is  shown  by  the  speed  and  uniformity  with  which  it 


STRUCTURE  AND  PROPERTIES  OF  THE  EARTH   261 

transmits  the  compressional  waves  of  earthquake  shock  in  any  direction  through 
its  mass.  Compared  with  the  vast  size  of  the  globe,  these  shocks  are  relatively 
very  feeble,  and  that  they  should  be  transmitted  such  great  distances  through 
it  is  a  striking  testimonial  to  the  elastic  nature  of  its  interior.  That  the  earth 
transmits  distortional  waves  of  earthquake  shock,  as  explained  under  earth- 
quakes, is  also  a  proof  that  its  interior  is  not  liquid,  or,  at  least,  that  it  pos- 
sesses the  physical  quality  of  elasticity  we  associate  with  a  solid,  and  not  with  a 
liquid. 

It  has  been  suggested  that  the  greater  elasticity  of  the  interior  of  the  earth  is 
due  to  increasing  density,  caused  by  the  tremendous  pressure  of  the  superin- 
cumbent material.  It  is  calculated  that  the  pressure  at  the  center  is  equal  to 
3,000,000  atmospheres,  or  45,000,000  Ibs.  per  square  inch,  and,  of  course,  it  varies 
with  the  depth;  at  the  surface  it  is  one  atmosphere,  at  one-fifth  the  radius,  or 
about  800  miles  down,  it  is  over  500,000  atmospheres,  or  7,500,000  Ibs.  per  square 
inch.  We  can  scarcely  conceive  that  such  pressures  would  not  cause  an  in- 
crease in  density,  especially  towards  the  center,  and  the  transmission  of  earth- 
quake shocks,  as  mentioned  above;  indicates  that  the  difference  in  density  and 
elasticity  is  gradual,  and,  perhaps,  due  to  increasing  pressure  alone,  but  it  may 
also  depend  on  a  difference  in  the  kind  of  material  forming  the  inner  and 
outer  portions. 

From  the  high  specific  gravity  of  the  earth,  and  its  magnetic  properties,  it  is 
thought  by  some  that  the  interior  consists  largely  of  iron,  and  the  fact  that 
most  of  the  material  which  comes  to  us  from  space  in  the  form  of  meteors  is 
composed  of  this  metal,  is  held  to  strengthen  this  view. 

Interior  Heat  of  the  Earth. — This  subject  has  been  mentioned 
under  volcanoes,  and  may  be  now  further  discussed.  The  interior 
heat  manifests  itself  directly  in  two  evident  ways,  by  the  uprise 
and  outflow  of  molten  magma  and  heated  vapors,  and  by  the  in- 
crease of  temperature  as  one  descends  into  the  earth.  Indirectly  its 
presence  is  also  indicated  by  certain  changes  and  phenomena,  to  be 
discussed  later,  which  have  occurred  in  the  rock-shell  of  the  earth, 
and  for  which  the  presence  of  heat  has  been  necessary. 

The  rise  of  temperature  as  one  penetrates  the  rock-shell  varies  in 
different  regions.  The  average  is  stated  as  1°  F.  for  every  60  feet, 
but  this  means  little  from  the  practical  point  of  view,  for  the  in- 
crement may  be  much  greater  or  less  than  this.  In  the  region  of 
the  copper  mines  of  Michigan,  which  are  nearly  5,000  feet  deep,  it 
is  about  1°  F.  for  every  100  feet,  while  in  mines  in  other  places  it 
may  be  almost  five  times  as  rapid. 

In  the  relatively  shallow  depths  reached  by  mines  it  may  be  much 
influenced  by  local  conditions,  thus  in  some  the  rapid  increment  noted  may 
be  due  to  chemical  processes,  such  as  the  oxidation  of  ores  containing 
sulphur,  while  the  slow  rise  of  temperature  noted  above  in  the  Michigan 
copper  mines  may  be  partly  induced  by  the  greater  conductivity  of  the  rocks 
of  that  area.  Also  these  rocks  are  very  old  ones  geologically,  and  it  has 
been  noted  that  in  old  rocks  the  rise  is  slow,  perhaps  100  feet  or  more  to  1°F., 


262  TEXT-BOOK   OF   GEOLOGY 

whereas  in  young  volcanic  rocks  it  may  be  comparatively  rapid,  28  feet  to 
1°F.  In  Great  Britain  it  is  stated  to  vary  from  34  to  130  feet.  It  is  often 
a  matter  of  great  importance  in  mining  and  tunneling  operations:  thus 
some  mines  have  been  found  difficult  and  expensive  to  work  owing  to  the 
great  heat  encountered,  and  it  is  thought  that  on  this  account  mining  below 
certain  depths  would  not  be  feasible.  In  the  making  of  the  great  railway 
tunnels  which  traverse  the  Alps  this  factor  has  been  a  serious  one;  the 
surfaces  made  by  connecting  points  of  equal  heat  increment  as  one  goes 
down,  and  .known  as  isogeotherms,  are  not  necessarily  parallel  to  the  general 
surface  of  the  earth,  but  more  or  less  irregular,  and  rise  in  the  interior  of 
mountain  chains,  so  that  in  a  horizontal  tunnel  one  encounters  greater  heat 
as  the  tunnel  proceeds  inward.  See  Fig.  201.  Thus,  although  the  economy 
of  operation  on  lower  grades  would  more  than  offset  the  cost  of  driving 
longer  tunnels  at  lower  levels,  the  interior  heat  prohibits  their  construction 
below  certain  levels,  and  in  one  recently  constructed  it  was  so  great  there 
was  fear  for  a  time  that  the  undertaking  would  have  to  be  abandoned. 


Fig.  201.  —  Illustrating  the  rise  of  heat  in  the  interior  of  mountains,  and  the  difference 
in  the  difficulty  of  constructing  high  and  low  level  tunnels. 

If  the  heat  increased  regularly  1°  F.  for  every  60  feet  of  descent 
there  would  result  a  temperature  of  3,600°  F.  at  41  miles;  this 
would  liquefy  rocks  at  the  surface,  especially  if  the  melt  contained 
water  vapor,  and  it  is  probably  as  high  a  temperature  as  that  of 
the  hottest  lavas,  which  attain  the  surface,  or  higher.  As  shown  by 
volcanic  phenomena  we  can  reasonably  assume  that  there  are 
regions  within  the  earth  where  the  temperature  attains  a  height 
of  at  least  3,600°  F.  =  about  2,000°  C.,  but  beyond  this  all  is  un- 
known and  a  matter  of  speculation.  The  depth  of  41  miles  is  also 
an  assumption,  for  it  is  based  on  the  rate  of  1°  F.  for  60  feet; 
whereas  the  average  rate,  after  some  depth  is  attained,  may  be 
quite  different. 

If  one  assumes  that  the  rate  of  heat  increase  mentioned  above  is  uniform  to 
the  center  of  the  earth,  the  temperature  would  there  be  about  350,000°  F.,  but 
for  several  reasons  this  seems  highly  improbable.  The  outermost  shell  of  the 
earth  is  a  poor  conductor  of  heat,  the  inner  portions  with  higher  density  should 
be  good  conductors;  if  we  should  imagine  that  at  some  depth  below  the 
surface  it  is  relatively  hot,  say  2,000°  C.,  the  fall-off  in  heat,  or  rate  of 
decrease,  in  the  outer  part  towards  the  surface  would  be  rapid;  while  in  the 
other  direction  from  this  point  towards  the  interior  we  should  expect  the 
rise  in  temperature,  with  increasing  density  and  better  conduction,  to  be 
relatively  slow. 


STRUCTURE    AND    PROPERTIES    OF    THE    EARTH        263 

Nature  of  the  Earth's  Interior :  Origin  of  Heat.  —  Our  ideas  of 
the  cause  of  the  interior  heat  of  the  earth  must  of  necessity  be 
closely  connected  with  those  regarding  the  nature  of  its  interior,  and 
these  in  turn  lead  to  speculation  concerning  the  origin  of  the  earth 
itself.  The  last  subject  is  historical  in  character  and  is,  therefore, 
properly  treated  in  the  second  portion  of  this  work.  In  their  bear- 
ing on  the  question  of  the  interior  structure  of  the  globe  and  its 
heat,  some  prominent  views  which  have  been  advanced  are,  how- 
ever, of  importance,  and  may  be  briefly  considered. 

a.  Nebular  hypothesis.  The  view  which  has  been  long,  and  is  still  widely 
held,  is  that  the  earth,  formed  as  the  condensation  of  a  portion  of  a  vast 
glowing  cloud  of  extended  vapor,  was  once  a  molten  mass,  whose  outer  shell 
through  cooling  solidified  as  a  solid  crust,  while  the  interior,  though 
excessively  hot,  also  solidified  through  the  enormous  pressure  of  the  superin- 
cumbent layers;  and  that  between  the  two,  is  either  a  zone  of  liquid,  because 
the  pressure  there  is  not  sufficient  to  solidify  it,  or  one  of  material  solidified  by 
pressure,  but  so  hot  that,  if  in  any  way  the  pressure  is  sufficiently  diminished, 
it  will  liquefy.  According  to  this  view  the  heat  of  the  earth  is  primitive; 
what  it  now  exhibits  is  that  remaining  from  its  former  condition.  It  may 
be  remarked  in  regard  to  this  hypothesis  that  the  supposition  of  a  liquid 
layer  is  no  longer  tenable  in  view  of  what  has  been  learned  concerning  the 
rigidity  and  elasticity  of  the  earth,  as  previously  shown;  if  the  general 
hypothesis  with  the  second  alternative  be  accepted,  the  zone  where  liquefaction 
will  ensue,  if  pressure  be  sufficiently  relieved,  becomes  of  geologic  importance 
and  will  be  further  considered  elsewhere. 

A  modification  of  the  above  hypothesis  consists  in  the  assumption  that 
the  increase  of  heat  is  so  great  that  towards  the  center  matter  cannot  remain 
in  either  the  solid  or  liquid  condition,  but,  being  heated  above  the  critical 
point,  it  must  be  in  the  gaseous  form  and,  on  account  of  the  enormous 
pressure,  contracted  to  a  density  far  beyond  that  of  solids  at  the  surface. 
By  reason  of  this  condensation  the  substances  are  supposed,  although  in  the 
gaseous  condition,  to  possess  so  great  an  internal  viscosity,  or  resistance  to 
flowage,  that  the  mass  has  a  rigidity  sufficient  to  meet  the  demands  which, 
as  has  been  shown  above,  astronomical  considerations  impose.  It  is  inferred 
that  certain  facts  concerning  the  transmission  of  earthquake  shocks  favor 
this  view.  It  is  to  be  noted  that,  if  the  core  increases  so  greatly  in  density, 
the  outer  shell  of  low  specific  gravity  must  be  of  considerable  thickness 
in  order  that  the  average  density  of  the  whole  earth  may  be  maintained 
at  the  proper  figure  of  5.6,  and  this  appears  to  agree  with  the  results  of 
seismic  investigation,  previously  stated.  Under  the  conditions  and  with  the 
properties  assumed,  the  term  "gaseous"  seems  hardly  applicable.  With  the 
enormous  pressures  reigning  at  the  center  of  the  earth,  the  condition  of 
matter  must  be  very  different  from  anything  known  at  the  surface,  whether 
it  be  moderately,  or  enormously  hot.  Under  sufficient  pressure  and  proper 
conditions  the  rigidity  of  solid  metals  at  the  surface  of  the  earth  breaks 
down,  and  they  undergo  through  mashing  a  flowage  like  liquids.  But  the 
resistance  to  flowage  is  greater  than  would  be  the  rigidity  at  the  earth's 
surface.  Under  such  pressures  it  would  seem  as  if  substances  would  be 


264  TEXT-BOOK  OF  GEOLOGY 

resolved  into  a  condition  neither  solid,  liquid,  nor  gaseous,  as  we  know  them, 
and  which  might  be,  indeed,  a  fourth  state  of  matter;  the  condition  they 
might  assume  on  relief  of  pressure  may  depend  on  the  temperature. 

b.  Planetesimal  hypothesis.    In  recent  years  serious  objections  have  been 
advanced  which  throw  doubts  upon  the  validity  of  the  nebular  hypothesis 
in  the  form  previously  stated  and  another  hypothesis  has  been  propounded  in 
the  endeavor  to  meet  them.    The  statement  of  this  is  found  in  the  second 
part  of  this  book;   it  is  sufficient  to  say  here  that  the  earth  is  regarded 
as  having  been  built  up  gradually  by  the  infall  and  accretion  of  relatively 
small  solid  bodies  termed  "planetesimals."    Through  the  enormous  pressures 
exerted  under  the  influence  of  gravity,  contraction  has  ensued,  and  gaseous 
matters  have  been  expelled,  giving  rise  to  the  atmosphere  and  surface  waters. 
The  contraction  is  thought  to  be  the  source  of  the  interior  heat;   in  the 
interior  core,  where  the  contraction  is  greatest,  the  most  heat  develops,  and 
this  flows  outwardly  to  an  intermediate  zone.    The  latter  is  held  to  receive 
heat  faster  from  the  interior  than  it  loses  it  by  conduction  to  the  outer 
crustal  zone;   as  a  result  melting  ensues,  and  the  liquid  material,  by  the 
forces  to  which  it  is  subjected,  works  its  way  upward  to  the  surface  and, 
along  with  the  escaping  gases,  gives  rise  to  volcanic  phenomena.    The  escape 
of  heat  through  volcanic  agencies  regulates  the  temperature  of  the  interme- 
diate zone,  and  prevents  any  notable  mass  of  it,  beyond  relatively  thin  vol- 
canic threads,  from  becoming  liquid.    This  hypothesis  has  been  recently  ad- 
vanced;  it  apparently  meets  the  objections  raised  against  the  former  one; 
whether  it  will  encounter  new  difficulties  of  its  own,  time  alone  can  tell. 

c.  The  radio-active  properties  of  matter  have  still  more  recently  been  ap- 
pealed to  as  a  source  of  the  earth's  interior  heat,  as  has  been  previously  men- 
tioned under  the  causes  of  volcanic  energy. 

We  have  learned  that  in  the  disintegration,  or  breaking  down  of  certain  ele- 
ments, such  as  uranium  and  thorium,  into  other  elements,  such  as  radium  and 
lead,  and  in  the  further  disintegration  of  radium  into  helium  and  by-products, 
relatively  enormous  quantities  of  heat  are  developed.  Furthermore,  radio- 
activity is  found  to  be  a  widely  spread  property  of  rocks,  especially  of  the  igne- 
ous ones.  That  a  part  of  the  earth's  interior  heat  is  due  to  this  cause  is  un- 
questionable;  recent  investigations  would  seem  to  indicate  that  it  may  be 
entirely  so.  It  also  seems  most  probable  that  radium,  and  the  heat  which  it 
produces  by  its  disintegration,  are  confined  to  a  shallow  zone  of  the  exterior, 
but  a  few  miles  in  depth.  It  is  stated  that  investigation  in  the  Alpine  tunnels 
has  shown  that  the  rate  of  heat-increase  is  proportional  to  the  radio-activity  of 
the  rocks.  While  we  are  not  yet  in  a  position  to  see  clearly  the  full  significance 
of  the  matter,  it  seems  probable  that  this  conception  of  the  part  played  by 
radio-activity  may  lead,  as  it  is  further  investigated,  to  ideas  concerning  the 
physical  state  of  the  earth's  interior,  and  the  cause  of  its  different  energies, 
which  are  quite  different  from  anything  expressed  in  the  foregoing  discussion. 

Isostasy.  —  This  term  (from  the  Greek,  meaning  equal  stand- 
ing) is  applied  to  a  theory  of  the  physical  condition  of  the  globe  in 
which  it  is  conceived  to  be  in  such  a  state  of  relative  plasticity, 
either  through  the  viscous  yielding  of  material,  or  the  forced  flow  of 
solid  matter,  such  as  rocks,  through  gravitational  pressures,  that 


STRUCTURE  AND  PROPERTIES  OF  THE  EARTH   265 

from  circumference  to  center  each  column  of  substance  composing 
it,  like  the  spokes  of  a  wheel,  is  in  a  sort  of  hydrostatic  equilibrium 
with  every  other  column.  That  is,  columns  of  equal  diameter  have 
the  same  weight.  But  as  some  parts,  like  the  continents,  stand  at 
a  higher  level  than  other  parts,  like  the  deep  ocean  basins,  they  must 
do  so  because  the  material  composing  them,  in  the  top  part  of  their 
column,  is  deficient  in  density  compared  with  the  other  parts  below 
them  in  level.  See  Fig.  185.  As  the  outer  shell  of  the  earth  is 
composed  of  heterogeneous  substances,  and  through  erosion  and 
other  processes  constant  shifting  of  material  is  going  on,  the  theo- 
retical condition  supposed  above  is  not  perfect,  and  the  earth  is  con- 
stantly tending  to  bring  itself  into  isostatic  equilibrium  by  the  slow 
flowage  of  material  below,  through  the  stresses  produced  by  gravity. 
According  to  this  view  the  continental  masses  are  the  result  of  a 
kind  of  relative  flotation  of  lighter  material  below  their  surfaces 
and,  therefore,  project,  whereas  the  ocean  basins  are  depressed 
because  of  the  denser  material  below  them. 

Aside  from  the  general  geologic  evidence  that  those  areas,  where  lightening 
of  the  crust  by  erosion  is  taking  place,  are  rising  ones,  while  those  parts  of 
the  sea-floor  where  rivers  are  laying  down  heavy  deposits  are  sinking  (see 
page  239),  this  theory  receives  some  additional  support  from  the  results  of 
surveys  made  by  the  U.  S.  Government  to  determine  the  form  and  curvature 
of  the  United  States,  and  from  pendulum  experiments  to  ascertain  in 
different  regions  the  force  of  gravity.  These  show  that  there  is  a  general 
isostatic  balance  between  the  continents  and  ocean  basins,  but  that  the  large 
mountain  ranges  are  probably  not  in  isostatic  equilibrium. 

It  has  been  calculated  that  at  a  depth  of  about  70  to  100  miles  isostatic 
compensation  is  complete,  and  that  the  adjustments  take  place  in  this  upper 
shallow  zone.  The  variation  in  density  between  the  highest  large  area  on 
the  continent,  the  Colorado  Plateau  (reaching  to  11,000  feet),  and  the 
greatest  ocean  depth  of  the  Atlantic  (18,000  feet),  which  have  been  investi- 
gated, is  actually  small,  only  about  3  per  cent  less,  or  greater,  than  the 
normal  one  at  sea-level,  and  less  than  the  difference  between  different  kinds 
of  rocks.  This  seems  in  agreement  with  the  general  observation  that  the 
rocks  composing  the  continental  masses  are  of  lighter  specific  gravity  than 
the  basaltic  ones  which  the  mid-oceanic  volcanoes  usually  contain,  and 
which  latter  are  our  clue  as  to  the  nature  of  the  material  underlying  the 
ocean  floors.  It  should  be  said,  however,  that,  although  the  experimental  work 
mentioned  shows  that  some  sort  of  compensation  is  probable,  the  exact 
results  which  it  was  thought  to  yield  have  been  considered  to  be  very  doubtful. 
Further  work  in  this  direction  is  needed  before  we  shall  be  able  to  consider 
the  geologic  evidence  fully  supported  by  the  mathematical  results  obtained 
by  physical  measurements. 

It  may  at  first  thought  seem  contradictory  to  what  has  previously  been 
stated  concerning  the  rigidity  of  the  earth,  that  it  should  yield  to  the  com- 
paratively small  loadings  and  unloadings  which  isostatic  equilibrium  would 


266  TEXT-BOOK   OF   GEOLOGY 

imply,  or,  in  other  words,  that  it  is  so  plastic  and  so  weak  a  structure. 
But  the  student  must  remember  that  all  such  terms  are  relative  and  that 
the  earth,  while  adjusting  itself  to  minor  differences  of  load  and  level,  may 
yet  be  sufficiently  strong  to  resist  the  stresses  which  astronomical  considera- 
tions show  the  sun  and  moon  impose  upon  it. 

The  theory  of  isostasy  may  prove  of  importance  in  enabling  us  to  under- 
stand certain  geological  processes,  but,  as  it  has  not  yet  been  sufficiently 
investigated  to  receive  general  acceptance,  and  certain  facts  which  appear 
opposed  to  it  have  not  yet  been  explained,  it  should  be  regarded  at  present 
as  tentative. 

Relief  Form  of  the  Earth 

General  Features.  —  The  irregularities  of  the  earth's  surface,  or 
its  relief,  divide  naturally  into  major  and  minor  groups.  The 
former  are  the  continental  masses  and  ocean  basins,  while  the 
minor  groups  consist  of  mountains  as  opposed  to  interior  valleys 
and  basins  on  the  land,  and  of  islands  as  contrasted  with  the  deeps 
on  the  sea-floor.  The  mean  height  of  all  the  lands  above  sea-level 


Fig.  202.  —  Mt.  Everest  in  comparison  to  the  size  of  the  globe.  Part  of  the  arc  of  a 
globe  with  radius  of  one  foot  is  shown;  on  this  Mt.  Everest,  E,  would  be  about  ifo 
of  an  inch  high.  D,  in  a  similar  way,  shows  the  greatest  depth  of  the  ocean. 

is  about  2,400  feet,  of  North  America  about  2,000  feet;  the  average 
depth  of  the  sea  about  13,000  feet.  The  highest  elevation  of  the 
land,  Mt.  Everest  in  the  Himalayas,  is  29,000  feet;  the  lowest 
known  point  in  the  ocean,  in  the  Pacific,  is  31,000  feet  deep.  This 
makes  the  greatest  difference  in  relief  60,000  feet  or  nearly  12  miles. 
Relative  to  the  size  of  the  globe  its  relief  is  extremely  small  and  it 
is,  therefore,  comparatively  smooth;  see  Fig.  202  which  shows  its 
greatest  roughness.  The  features  of  the  land  are  divided  into 
plains,  such  as  the  Atlantic  coastal  plain ;  plateaus,  such  as  that  of 
the  Colorado ;  and  mountains,  like  the  Appalachians  extending  from 
Canada  to  Georgia  and  Alabama.  In  regard  to  the  grouping  of 
the  relief  forms  of  the  earth,  certain  facts  are  of  interest  and  impor- 
tance. The  continents  have  a  tendency  to  consist  of  interior  basins 
with  mountain  chains  as  coastal  rims,  while  the  ocean  basins  often 
reverse  this  with  deeps  near  the  continents,  and  submarine  ridges, 
or  up-swells  of  the  bottom,  in  mid-ocean.  And  in  a  number  of 
cases  the  highest  and  most  important  ranges  on  the  edge  of  a  con- 
tinent border  the  important  deeps  in  the  ocean  floor,  as  for  instance 
the  Andes  in  South  America  and  the  partly  submerged  mountain 
chain  which  forms  the  Japanese  islands,  and  is  the  real  eastern 


STRUCTURE    AND    PROPERTIES    OF    THE    EARTH        267 


268  TEXT-BOOK  OF  GEOLOGY 

border  of  the  continent  of  Asia;  close  to  these  the  ocean  floor  de- 
scends to  great  depths.  See  page  250.  This  is  not  meant  to  imply, 
however,  that  mountains  are  found  only  at  the  continental  edges, 
for  they  may  extend  in  a  wide  zone  far  into  the  interior,  as  in  west- 
ern North  America,  or  form  systems  crossing  a  continental  mass,  as 
in  Asia. 

Character  of  North  America.  —  North  America  is  the  most 
typical  of  the  continents  in  that  it  is  bordered  by  mountainous 
tracts  on  either  side  and  contains  the  great  basin  of  the  Mississippi 
and  its  tributaries  in  the  interior.  The  following  broad  features 
of  the  continent,  and  especially  of  the  United  States,  the  student 
will  do  well  to  bear  in  mind,  as  they  enter  into  many  of  the  dis- 
cussions of  its  geology.  On  the  east  and  south  the  continental 
shelf  rises  from  the  sea  as  the  Atlantic  coastal  plain,  and  this  meets 
the  base  of  a  rugged  mountainous  tract  of  country,  which  stretches 
from  Alabama  into  Canada  and  is  known  as  the  Appalachian  High- 
lands; it  includes  the  Appalachian  Mountains.  This  gives  way  to 
the  interior  basin  whose  higher  western  part  forms  the  Great  Plains. 
The  lower  part  of  the  basin  is  the  Central  Lowland,  from  which  the 
Mississippi  descends  through  the  Coastal  Plain.  To  the  west  the 
Great  Plains  give  way  to  the  long  series  of  north  and  south  ranges 
which  form  the  western  back-bone  of  the  continent  and  are  grouped 
under  the  name  of  the  Rocky  Mountain  System.  Between  this 
and  the  Pacific  Mountain  System,  which  makes  the  western  rim 
of  the  continent  and  consists  of  the  Sierra  Nevada  and  Cascade 
mountains  with  the  Pacific  border  lands,  lie  the  Great  Basin  and 
the  Colorado  and  Columbia  plateaus.  These  relations  and  other 
minor  ones  are  seen  on  the  map,  Fig.  203. 

The  physiographic  divisions  of  the  United  States  and  southern  Canada 
recognized  by  physiographers  are  exhibited  on  the  map,  Fig.  203.  They  are 
classified  into  major  divisions,  shown  by  letters,  and  minor  provinces,  indi- 
cated by  numbers.  Their  names  are  given  in  the  following  list: 

Major  Divisions  Provinces 

A.  Laurentian  Upland. .    ......(  4i*  Laurentian  Plateau. 

Ag,  Superior  Upland. 


B.  Atlantic    Plain 


C.  Appalachian  Highlands. 


BI}  Continental  Shelf  (submerged). 

B2,  Coastal  Plain. 

Cx,  Piedmont  Province. 

C2,  Blue  Ridge  Province. 

C3,  Appalachian  Valley  Province. 

C4,  St.  Lawrence  Valley. 

C5,  Appalachian  Plateaus. 

C6,  New  England  Province. 

Cw,  Adirondack  Mountains. 


STRUCTURE    AND    PROPERTIES    OF    THE    EARTH        269 

Major  Divisions  Provinces 

(  DX,  Interior  Low  Plateaus. 

D.  Interior    Plains |  D2,  Central  Lowland. 

D3,  Great  High  Plains. 

E. ,  Ozark  Plateaus. 

E.  Interior  Highlands 1 '  ,        _ 

E  j  Ouachita  Province. 


F.  Rocky  Mountain  System, 


G.  Intermontane  Plateaus. 


Southern  Rocky  Mountains. 
F  ,  Wyoming  Basin. 


Fa,  Northern  Rocky  Mountains. 
Columbia  Plateaus. 
Colorado  Plateaus. 


G3,  Great  Basin  and  Range  Province. 
H±,  Sierra-Cascade  Mountains. 
H.  Pacific  Mountain  System. . .  \  H2,  Pacific  Border  Province. 

H3,  Lower  California  Province. 


The  Outer  Zone  of  the  Earth;  Rocks 

As  has  been  shown  in  the  preceding  discussions,  we  know  but 
little  regarding  the  interior  of  the  earth ;  chiefly  it  is  the  outer  zone 
of  rock  of  which  we  have  extensive  and  positive  information;  upon 
it  we  live  and  exert  our  activities;  we  penetrate  into  it  for  fuels 
of  various  kinds,  for  metals,  water,  building  material,  and  other 
things,  and  for  reasons  upon  which  the  physical  side  of  modern 
civilization  depends.  A  thorough  knowledge,  therefore,  of  the  com- 
ponent parts  of  this  shell  and  its  structure  is  of  the  highest  im- 
portance. The  component  parts  are  rocks,  and  we  shall  begin  our 
inquiry  by  a  study  of  the  different  kinds  of  rocks  and  the  varied 
modes  in  which  they  occur.  For  the  most  part,  as  we  shall  see,  the 
causes  which  have  produced  them  have  been  described  in  the  fore- 
going part  of  this  work ;  we  are  here  concerned  with  the  results. 

.  Definition  and  Classification  of  Rocks.  —  The  word  rock,  geo- 
logically speaking,  means  the  material  composing  one  of  the  indi- 
vidual parts  of  the  earth's  solid  shell.  In  ordinary  usage  a  rock 
means  something  hard  and  firm,  but,  geologically,  a  rock  may  be 
composed  of  a  soft  substance;  thus  a  bed  of  clay  or  volcanic  ash 
may  be  considered  a  rock,  as  well  as  a  mass  of  granite. 

According  to  their  mode  of  origin,  and  the  position  of  the  masses 
with  respect  to  the  earth's  shell,  and  to  each  other,  rocks  are  divided 
into  three  main  groups ;  the  sedimentary,  or  bedded  rocks,  formed  by 
the  deposition  of  sediments,  chiefly  by  water  (and  to  some  extent  by 
air) ;  the  igneous  rocks,  made  by  the  solidification  of  molten  ma- 
terial; and  the  metamorphic  rocks,  produced  from  the  preceding 
groups  by  certain  processes  which  have  wholly,  or  partly,  destroyed 


270  TEXT-BOOK  OF  GEOLOGY 

their  original  characters  and  replaced  them  by  new  ones,  so  that 
they  may  be  conveniently  considered  in  a  separate  group. 
Thus  we  have  three  groups: 

I.  Sedimentary  Rocks,  sediments  deposited  by  water  or  air. 
II.  Igneous  Rocks,  consolidated  molten  masses. 
III.  Metamorphic  Rocks,  secondary,  from  I  and  II. 


CHAPTER  XI 
SEDIMENTARY  ROCKS 

The  Composition  and  Character  of  Sedimentary  Deposits 

Sedimentation  and  Stratification.  —  If  material  of  various  de- 
grees of  fineness  be  dropped  into  still  water,  the  heaviest  and 
coarsest  particles  will  descend  and  reach  the  bottom  first.  Upon 
them  will  fall  the  next  in  size,  and  so  on  to  the  top  of  the  deposit, 
which  will  consist  of  the  finest  ones,  thus  making  a  regular  gradation 
from  bottom  to  top.  If  the  water,  instead  of  being  still,  were  moving 
in  a  regular  current,  the  gradation  would  not  take  place  wholly  in  a 
vertical  direction,  but  in  a  horizontal  one  as  well,  the  successively 
finer  material  being  dropped  farther  and  farther  along  the  bottom. 
This  material  would  be  graded,  but  not  stratified.  If,  however,  the 
process  be  repeated,  and  the  velocity  of  the  current  changed  even 
to  a  slight  extent,  since  in  a  foregoing  part  of  this  work  (page  42) 
it  has  been  shown  how  greatly  the  size  of  particles,  which  can  be 
carried  by  moving  water,  depends  on  the  velocity  of  the  current,  it 
will  happen  that,  although  the  new  deposit  will  be  graded  as  before, 
at  no  point  will  its  degree  of  fineness  exactly  correspond  with  that 
of  the  previous  layer  vertically  under  it.  The  two  layers  will  be 
separated  by  a  distinct  juncture  plane,  on  either  side  of  which  they 
will  differ  in  texture;  this  is  stratification,  and  the  juncture  plane  is 
called  a  bedding  plane.  It  is  clear  that  development  of  stratification 
is  favored  by  variation  in  size  of  particles  and  in  velocity  of  current. 
Now  as  all  currents,  whether  streams  on  the  land  or  tidal  ones  in  the 
sea,  are  constantly  varying  from  place  to  place  and  from  time  to  time, 
the  deposits  of  the  sediment,  which  they  may  carry  and  drop  as 
they  slacken,  are  always  distinctly  stratified,  that  is,  made  up  of 
parallel  layers,  or  beds,  which  may  differ  in  thickness,  texture,  and 
materials.  A  given  layer  may  be  part  of  an  inch,  or  a  hundred  feet 
or  more,  in  thickness,  and  it  represents  a  period  during  which  the 
conditions  of  deposition  were  uniform.  A  great  thickness  of  very 
fine  material  indicates  a  prolonged  interval  of  quite  regular  con- 
ditions, and  the  probability  of  the  deposit  having  taken  place  on  the 
sea-floor,  or  in  some  large  lake.  A  single  layer  is  known  as  a  bed,  or 
stratum,  and  a  close  examination  may  show  that  this  is  made  up  of 

271 


272  TEXT-BOOK  OF   GEOLOGY 

much  finer  layers,  which  may,  indeed,  be  as  thin  as  paper,  and  are 
known  as  lamince.  See  Fig.  204.  A  collection  of  beds,  lying  con- 
cordantly  above  one  another,  deposited  during  a  minor  geological 
division  of  tune,  and  with  similar  characters,  is  called  a  formation. 

Matter  carried  and  deposited  by  air  currents  may  also  be  stratified,  though 
generally  much  more  rudely  than  when  the  work  is  done  by  water.  Thus  in  a 
volcanic  outburst  the  ashes  driven  by  th'e  wind  may  spread  over  a  wide  extent 
of  country;  the  heavier  and  coarser  particles  fall  first,  to  be  succeeded  by 


Fig.  204.  —  Thin  laminae  composing  part  of  a  bed  of  sandstone,  natural  size. 
The  displacement,  or  fault,  has  occurred  since  deposition. 

finer,  and,  finally,  by  dust.  This  produces  gradation,  but,  if  a  new  outburst 
occurs,  the  coarser  particles  first  falling  will  rest  on  the  finer  of  the  previous 
eruption,  and  a  continuation  of  this  process  will  give  rise  to  stratified  beds  of 
volcanic  tuff,  as  may  be  seen  in  many  parts  of  the  Rocky  Mountains.  On  the 
other  hand,  deposits  of  loess  (page  18)  show  little  or  no  stratification,  indicat- 
ing general  uniformity  in  the  size  of  the  particles,  and  conditions  of  deposition. 

But,  while  deposits  made  by  the  wind,  such  as  drifted  sand  or 
volcanic  ashes,  are  sometimes  rudely  stratified,  these  are  of  small 
importance  compared  with  the  great  masses  of  material  which  are, 
and  have  been  in  times  past,  carried  and  laid  down  by  moving 
waters,  and,  as  shown  above,  such  exhibit  by  their  stratification  the 
manner  in  which  they  have  been  formed.  See  Fig.  205. 

Materials  Involved.  —  The  material  which  currents  are  able  to 
transport  and  deposit,  whether  upon  the  land  or  in  the  sea,  may 
have  one  of  two  modes  of  origin.  It  may  be  either  the  waste  of  the 
land,  or  matter  produced  by  life  in  the  sea.  The  first  may  be 
considered  mechanical,  the  second  organic  in  mode  of  formation. 
The  waste  of  the  land,  through  the  destruction  of  previously  existent 
rocks  by  various  erosive  processes,  and  its  transport  have  been 


SEDIMENTARY   ROCKS 


273 


already  treated  in  the  foregoing  part  of  this  work,  as  has  also 
the  production  of  lime  carbonate  deposits  by  living  organisms  in 
the  sea.  They  need,  therefore,  only  this  mention  to  show  the 
contrast  between  them,  one  kind  of  material  being  of  continental, 
the  other  of  marine  origin.  There  are  other  kinds  of  deposited 
material,  such  as  rock  salt,  but  these  are  of  such  minor  importance 
that  they  need  not  be  considered,  at  present,  in  this  connection. 


Fig.   205.  —  Regularly  bedded  sandstones  and   shales,   near  Pueblo,   Colo.     G.   K. 
Gilbert,  U.  S.  Geol.  Surv. 

The  land  waste,  according  to  the  size  of  the  pieces,  or  particles, 
is  roughly  graded  into  gravel,  sand  and  mud,  or  clay,  as  follows: 

Gravel.  —  This  is  composed  of  material  from  the  size  of  a  pea  up, 
and  the  individual  pieces  are  termed  pebbles;  large  loose  fragments 
of  rock  are  called  bowlders.  Pebbles  which  have  suffered  a  long 
transport  in  the  beds  of  streams,  or  have  been  much  rolled  by  waves 
on  the  shores  of  seas  and  lakes,  have  a  characteristic  rounded  ap- 
pearance. This  depends  also  on  the  hardness  of  the  substance  com- 
posing them.  The  mineral  quartz,  on  account  of  its  hardness  and 
durability,  is  one  of  the  commonest  substances  forming  pebbles,  but 
other  kinds  of  minerals  and  rocks,  such  as  granite,  basalt,  limestone, 
etc.,  are  frequent.  Sediments  are  sometimes  composed  of  pebble- 
sized  fragments  which  still  retain  their  rough,  angular  shapes;  in 
this  case  we  judge  that  they  have  suffered  little  movement  and  are 
not  far  from  their  place  of  origin. 

Sand.  —  Material  composed  of  particles  smaller  than  peas,  ancl 


274  TEXT-BOOK  OF  GEOLOGY 

yet  sufficiently  coarse  so  that  it  will  not  form  a  mass  cohering  when 
wet,  is  known  as  sand.  An  ordinary  sand  would  be  like  granulated 
sugar  in  fineness.  It  may  be  seen  with  a  lens  that  the  grains  of 
coarser  sand,  such  as  is  found  on  sea-beaches,  are  rounded  like 
pebbles  by  attrition,  but  in  the  finest  sands  they  may  be  angular. 
Quartz  so  commonly  forms  sand  that,  unless  otherwise  stated, 
quartz-sand  is  understood.  Many  other  minerals  are  found  in 
sands,  and  on  the  beaches  o'f  coral  islands  the  grains  may 
be  made  wholly  of  lime  carbonate. 

Mud,  Silt,  Clay.  —  This  is  the  finest  part  of  the  land  waste  and 
when  dry  it  may  form  dust.  It  coheres  when  wet.  As  a  sedi- 
mentary deposit  it  is  found  off-shore,  or  in  sheltered  parts  of  estu- 
aries, gulfs  and  bays  where  the  slow  movement  of  the  water  does 
not  permit  the  transport  of  the  heavier  sand  and  gravel,  or  on  the 
flood  plains  and  deltas  of  rivers.  As  quartz  is  the  characteristic 
mineral  of  sands,  so  is  kaolin  that  of  many  muds  and  especially 
clays;  as  shown  under  the  formation  of  soil,  it  is  made  by  the 
decay  of  the  feldspars  of  the  rocks.  In  the  destruction  of  the  latter, 
since  the  quartz  particles  are  heavier,  while  those  of  clay  are 
extremely  light  and  fine,  there  tends  to  take  place  a  separation 
of  the  two  by  moving  water;  the  quartz  grains  deposit  first,  form- 
ing sand,  while  the  light  clays  settle  later,  or  are  carried  beyond 
into  still  water. 

In  summation,  then,  it  is  seen  that  the  sedimentary  deposits  con- 
sist mainly  of  sand,  clays  and  carbonate  of  lime,  and  their  varied 
intermixtures;  gravels  are  of  less  importance  in  quantity,  though 
geologically  of  great  interest,  as  we  shall  see  later.  Carbonate  of 
lime  deposits  have  been  already  treated  in  the  chapter  on  the  work 
of  organic  life.  Volcanic  ash,  organic  matters  such  as  peat,  common 
salt,  and  gypsum  are  also  deposited  materials,  but,  although  of 
interest  and  of  local  importance,  they  are  not  of  the  same  geological 
consequence  as  those  mentioned  above.  For  glacial  deposits  see 
page  144. 

Places  of  Deposit 

The  places  where  moving  waters  may  deposit  sediment  can  be 
divided  into  three:  the  land,  the  beach  or  area  between  the  limits  of 
average  high  and  low  tide,  and  the  sea-floor.  They  may  thus  be 
classified  as  continental,  littoral,  and  marine  deposits.  Since  the 
distinction  between  these  is  a  matter  of  great  geological  importance 
they  must  be  considered  separately,  in  some  detail. 

Continental  Deposits.  —  The  formations  made  upon  the  land  by 


SEDIMENTARY   ROCKS  275 

moving  waters  may  be  divided,  according  to  their  origin,  into  the 
following  classes: 

Desert  Deposits  of  Arid  Regions. 
Piedmont  River  Deposits. 
Basin  Deposits  of  Humid  Regions. 
Sub-aerial  Delta  Deposits. 

Each  of  these  is  of  sufficient  consequence  to  demand  some  con- 
sideration, as  follows: 

Desert  Deposits.  —  It  has  been  previously  shown,  page  82  and 
following,  that  in  all  the  continents  interior  drainages  exist,  caused 
by  the  excess  of  evaporation  over  rainfall.  In  such  regions  the  land 
waste  of  the  slopes  of  the  basin  constantly  tends  to  move  toward  the 
more  central  parts  and  to  form  deposits.  It  may  be  moved  by  rivers 
into  permanent  lakes,  like  Great  Salt  Lake  and  the  Caspian  Sea, 
slowly  filling  them  up,  or  in  times  of  rainfall  temporary  streams  may 
spread  out  in  thin  sheet-floods  over  lower  level  areas,  giving  rise  to 
temporary  lakes  (playas),  in  whose  waters  the  sediments  brought 
down  may  settle.  Thus,  through  the  continued  action  of  rain-wash 
and  streams,  aided  by  wind  drift  in  times  of  dryness,  the  desert 
basins  tend  to  fill  up  by  deposits,  which  may  become  very  thick, 
though  at  times  and  in  places  a  reverse  action  through  the  export  of 
material  by  the  wind  may  thin  them.  Such  deposits  often  contain 
layers  of  salt  and  gypsum  as  characteristic  features,  for  reasons  ex- 
plained under  salt  lakes,  and  are  apt  to  have  a  red  coloration,  as 
explained  on  page  172.  See  also  work  of  the  wind,  page  13. 

Piedmont  River  Deposits.  —  Where  a  young  and  lofty  mountain 
range  is  undergoing  extensive  erosion,  it  may  happen  that  the  rivers 
draining  it  become  so  heavily  loaded  with  sediment,  that,  when  they 
issue  upon  the  piedmont  belt  (piedmont,  foot  of  mountain)  of 
country  below,  their  slackening  current  is  unable  to  carry  it  all,  and 
the  part  of  the  burden  in  excess  of  transporting  power  is  deposited. 
In  this  portion  of  its  course  a  river  may,  therefore,  be  aggrading, 
instead  of  eroding,  and  in  times  of  flood  its  deposits  may  be  widely 
spread  over  the  adjacent  country.  Through  the  continued  action 
of  this  process,  during  a  long  period  of  time,  extensive  deposits 
of  sands  and  clays  of  great  thickness  may  be  formed.  This  is  illus- 
trated by  formations  lying  upon  the  Great  Plains  and  other  areas 
of  country  at  the  foot  of  the  ranges  of  the  Rocky  Mountains'  tract 
in  western  North  America,  in  similar  ones  in  South  America  upon 
the  Pampas  east  of  the  Andes,  and  upon  the  piedmont  plain  of 
India  at  the  foot  of  the  Himalayas. 


276  TEXT-BOOK  OF  GEOLOGY 

It  used  to  be  considered  that  these  deposits  (of  the  so-called  Tertiary 
period)  in  western  North  America,  whose  fossils  indicate  them  to  be  of 
fresh-water  origin,  had  been  laid  down  in  extensive  lakes  then  existing,  but 
more  extended  study  has  shown  that,  while  in  part  this  view  may  be  true, 
it  is  not  a  necessary  one  to  explain  them,  since,  as  indicated  above, 
they  may  be  equally  well  formed  by  aggrading  rivers,  and  that  they  have 
been  made  for  the  greater  part,  if  not  entirely,  in  this  manner.  Ultimately, 
if  not  saved  by  some  intervening  geological  process,  as  subsidence  and 
covering  by  new  sediments,  such  deposits  in  their  turn  must  be  eroded  and 
carried  away  into  the  sea,  the  final  depot  of  land  waste. 

Basin  Deposits  of  Humid  Regions.  —  In  several  of  the  continents 
basin-like  depressions  occur  of  variable  extent  and  depth.  Where 
the  climate  is  arid,  and  the  rainfall  consequently  small,  these  may 
give  rise  to  interior  drainages,  as  previously  described.  But,  if  the 
rainfall  of  the  region  is  considerable  and  in  excess  of  evaporation,  as 
discussed  under  lakes,  these  depressions,  if  deep,  may  give  rise  to 
lakes,  such  as  the  Great  Lakes  of  North  America,  or,  if  very  shallow, 
to  wide  swampy  regions  covered  more  or  less  completely  at  times 
with  shallow  water,  like  the  basin  of  the  upper  Amazon  and  its 
tributaries.  Such  lake  basins  must  obtain  important  deposits  from 
inflowing  streams,  and  may  eventually  be  rilled  up,  while  the  shallow 
swampy  areas  receive  muds  and  clays  from  the  outstanding  waters 
of  flooded  rivers,  and  these  are  mingled  more  or  less  with  organic 
matter  from  the  decay  of  the  vegetation  which  flourishes  abundantly 
in  such  places.  This  swampy  condition,  with  resulting  accumula- 
tion of  river  sediment,  may  be  indefinitely  maintained  if  the  basin 
is  a  region  of  continued  subsidence.  Though  deposited  in  water, 
such  sediments  are  to  be  regarded  as  continental  in  origin,  since 
they  occur  in  hollows  of  the  land  surfaces.  Not  only  are  such 
deposits  forming  now,  but  they  have  been  made  extensively  in  times 
past,  as  we  shall  see  later. 

Delta  Deposits.  —  The  deltas  of  rivers  represent  so  much  land  re- 
claimed from  the  sea,  or  from  a  lake.  The  structure  of  the  deposits 
is  similar  in  both  cases,  except  that,  in  the  lake,  they  are  influenced 
by  feebler  waves  and  currents.  Lake  deposits  have  been  con- 
sidered in  the  foregoing  section;  what  follows  relates  chiefly  to 
deltas  formed  in  the  sea.  The  process  begins  by  the  deposition  of 
sediments  on  the  sea-floor ;  gradually  these  are  built  up  on  the  front 
of  the  advancing  delta  until  water-level  is  reached,  and  they  then 
become  land.  The  low-lying  land  is  flooded  in  times  of  high  water 
and  more  material  laid  down,  and  this  continues  until,  in  a  vertical 
direction,  a  balance  is  reached  between  upbuilding  in  periods  of 
highest  flood  and  erosion  at  other  times.  Meanwhile,  in  a  horizontal 


SEDIMENTARY    ROCKS 


277 


direction,  the  delta  is  advancing  seaward.  Thus  we  see  that  a  delta 
consists  of  a  mingling  of  marine,  littoral,  and  continental  deposits, 
and  this  is  because  it  is  situated  in  the  debatable  zone  where  land 
and  sea  struggle  for  mastery.  The  structure  of  a  delta  is  shown  in 
Fig.  206.  The  finest  material  is  carried  farther  out  and  forms  beds 


Water-level 


Fig.  206.  —  Illustrating  section  of  delta  built  out  in  quiet  water,  of  constant  level. 
A,  bottomset  beds;  B,  foreset  beds;  C,  topset  beds.     After  Barrell. 

horizontal,  or  nearly  so,  which  are  known  as  the  bottomset  beds. 
Down  the  slope  of  the  advancing  delta  are  dropped  the  coarser  sedi- 
ments, which  make  the  inclined  foreset  beds.  As  stated  above, 
deposition  also  happens  on  top  of  the  delta,  forming  the  topset  beds, 
which  are  horizontal,  or  nearly  so.  Thus  the  foreset  and  bottomset 
beds  are  marine  deposits,  the  topset  beds  may  be  largely  land,  or 
continental  ones,  while  the  littoral  or  beach  zone  is  of  minor 
importance.  The  deltas  of  great  rivers,  like  those  of  the  Mississippi 
and  Nile,  are  built  out  in  epicontinental  seas  on  the  submerged  con- 
tinental platforms,  and,  in  such  cases  of  wide  extent,  the  difference 


Water-level  after  gradual  subsidence 


Fig.  207.  —  To  illustrate  conditions  in  a  subsiding  delta.  A  portion  of  the  delta  built 
during  a  period  of  stationary  water-line  shows  below.  Through  subsidence  the 
water-line  has  advanced  to  the  left.  The  topset  beds  are  partly  subaerial  and 
partly  submarine  in  origin.  The  foreset  and  the  bottomset  beds  are  relatively  thin; 
compare  with  Fig.  206.  After  Barrell. 

of  angle  of  slope,  and  the  distinction  between  the  foreset  and  bot- 
tomset beds,  mostly  disappears.  It  has  often  happened  that  such 
shallow  seas  in  times  past  have  been  filled  by  delta  deposits,  which 
are,  therefore,  as  we  shall  see  later,  of  great  geologic  importance.  It 
has  already  been  mentioned  in  discussion  of  subsidences  of  the  earth's 
crust,  that  the  deltas  of  large  rivers  are  commonly  areas  of  sub- 
sidence, and  this,  as  shown  by  the  thickness  of  sediments  exposed, 
has  frequently  happened  in  the  past.  When  a  delta  is  subsiding, 
in  contrast  to  one  which  is  stationary,  the  upbuilding  in  a  vertical 


278  TEXT-BOOK  OF  GEOLOGY 

direction  increases  at  the  expense  of  its  horizontal  extension;  if  the 
movement  is  pronounced  the  latter  may  be  small. 

In  a  subsiding  delta  the  river  currents  tend  to  become  more  sluggish  through 
decreasing  grade,  and  the  flooding  of  it  more  frequent.  This  results  in  a 
greater  increase  of  material  dropped  upon  the  topset  beds;  the  latter  may 
thus  become  the  chief  contributors  to  the  delta  growth.  If  the  area  of 
the  latter  is  great,  it  may  thus  happen  that  the  volume  of  land  deposits, 
formed  by  the  topset  beds,  may  be  vastly  greater  than  that  of  the  foreset 
beds  which  build  out  the  delta's  front.  This  relation  is  shown  in  Fig,  207. 
Consequently  a  delta  which  is  growing  upward  because  of  a  subsiding  founda- 
tion tends  to  form  dominant  topset  beds  of  continental  nature;  a  delta 
growing  outward  because  of  a  stationary  one  tends  to  form  a  greater  volume 
of  subaqueous  beds. 

Littoral  or  Beach  Deposits.  —  The  beach  is  denned  as  the  area 
lying  between  average  high  and  low  tides.  If  the  slope  of  the  land 
to  the  sea  is  sharp,  it  may  be  a  very  narrow  zone,  or  in  the  case  of 
sea-cliffs  be  wanting.  If  the  inclination  of  the  land  is  very  gradual, 
it  may  be  of  wide  extent,  and  consist  chiefly  of  areas  of  salt  marshes 
and  tidal  lagoons,  exposing  mud  flats  at  low  tide.  Such  are  well 
shown  in  the  shallow  sounds  of  North  Carolina  back  of  the  barrier 
beaches,  and  in  the  wide  estuaries  of  Delaware  and  Chesapeake 
bays.  See  page  108.  Over  these  areas  sands  and  muds  are  laid 
down  as  deposits ;  but  generally  along  the  shore,  where  the  waves  are 
cutting  into  the  land,  and  the  beach  or  littoral  zone  is  very  narrow, 
and  exposed  to  the  rush  of  the  waves  and  tidal  currents,  only  the 
coarser  materials  such  as  gravel  and  sand  are  able  to  accumulate. 
Coarse  sand  and  gravel,  then,  are  the  most  characteristic  features  of 
beach  deposits,  and  they  cannot  be  of  great  thickness,  for,  if  the 
land  is  building  out  into  the  sea,  they  must  give  place  to  land 
deposits  and  be  buried  under  them,  or,  if  the  sea  is  encroaching  on 
the  land,  they  must  yield  to  marine  sediments  and  be  covered  by 
them.  The  importance  of  this  will  be  seen  when  the  geological 
structure  known  as  a  nonconformity  is  discussed  later. 

Marine  Deposits.  —  The  most  active  region  for  the  deposit  of 
land  waste  on  the  sea-floor  is  in  the  shallow  water,  extending  from 
the  average  limit  of  low  tide  out  to  the  depth  of  100  fathoms  and 
thus  upon  the  continental  shelves,  and  also  in  the  basins  of  epeiric 
seas  (page  111).  Over  these  areas  the  deposits  are  largely  ter- 
rigenous (of  land  origin),  consisting  chiefly  of  sands  and  muds 
brought  into  them  by  rivers,  or  formed  by  the  waves  gnawing  on 
the  coasts.  The  finer,  lighter  muds  tend  to  extend  farther  out  into 
deeper  water,  and  may  be  met  200  miles  from  land  extending  down 
the  slopes  of  the  ocean  basins.  These  marine  deposits  have  already 


SEDIMENTARY   ROCKS  279 

been  described  under  the  work  of  the  ocean,  page  111  and  following, 
and  need  no  further  mention. 

Enormous  deposits,  chiefly  of  carbonate  of  lime,  have  also  ac- 
cumulated on  the  sea-floor  in  times  past  through  the  agency  of 
living  organisms,  and  are  forming  at  the  present  time.  The  char- 
acter of  these  deposits,  and  the  conditions  necessary  to  produce 
them,  have  been  stated  under  the  geological  work  of  organic  life, 
page  190  and  following.  It  need  be  only  remarked  that  the  occur- 
rence of  such  deposits  is,  in  general,  indicative  that  the  seas  in  which 
they  were  laid  down  were  of  clear  water  and  possessed  moderate  to 
warm  temperatures, 

Consolidation  of  Sediments.  —  The  stratified  rocks  are  com- 
monly in  a  very  different  condition  from  that  in  which  they  were 
laid  down  as  sediments.  Were  it  not,  indeed,  proved  by  the  strati- 


Fig.  208.  —  Section  of  sandstone  (quartzite)  under  the  microscope.  The  dotted  areas 
are  the  rounded  sand  grains;  the  clear  ones  the  silica  deposited  about  the  grains, 
binding  them  into  rock. 

fication,  the  contained  fossils,  and  other  features,  it  would  be  difficult 
in  many  cases  to  recognize  their  origin;  to  perceive  in  hard  dense 
rocks  what  were  originally  soft  clays  and  sands.  The  causes  of 
consolidation  are  many,  and  often  complex.  For  one  thing,  there  is 
the  long- continued  and  heavy  pressure,  exerted  by  masses  which 
may  be  many  thousand  feet  in  thickness.  Another  important  factor 
is  the  deposition  of  material  from  solution  in  the  spaces  between  the 
grains,  which  cements  them  together.  There  is,  in  greater  or  less 
degree,  a  constant  leaching  of  material  from  the  upper  layers  by  per- 
colating waters,  and  a  transfer  and  deposition  of  it  at  lower  levels. 
The  most  common  cementing  substances  thus  deposited  in  the 


280 


TEXT-BOOK   OF   GEOLOGY 


rock-pores  are  carbonate  of  lime,  silica,  and  oxide  of  iron.  The  in- 
terstices may  become  almost  entirely  filled  with  cement,  as  illus- 
trated in  Fig.  208,  and  the  sediments  thus  converted  into  very  firm 
solid  rock. 

The  interior  heat  of  the  earth  rising  into  such  masses  of  sediments 
may  aid  in  some  degree  to  consolidate  them  by  quickening  the  chem- 
ical and  mechanical  activity  of  the  diffused  waters  which  deposit 
the  cement.  And,  finally,  since  the  conversion  of  sediments  into 
rock  must  be  a  slow  process,  time  is  an  important  element  in  the 
case.  Thus  we  observe  that,  in  general,  where  the  more  recent 
sediments  have  been  converted  into  land  surfaces,  they  exhibit  much 
softer  and  more  friable  stratified  rocks  than  the  older  ones.  It 
must  not  be  understood,  however,  that  this  process  of  cementation 
takes  place  only  under  the  sea,  for  on  land  also  the  same  process  of 
solution,  transfer  to  lower  levels,  and  redeposition  can  be  going  on. 

Kinds  of  Sedimentary  Rocks 

The  different  kinds  of  sedimentary  rocks  depend  upon  the  nature 
of  the  sediments  from  which  they  are  formed,  and  the  degree  of 
consolidation,  or  compactness,  which  they  have  assumed.  Thus, 
calcareous  muds  on  drying  may  form  a  chalk,  through  pressure  and 
cementation  they  become  limestone,  while  the  latter  through  cer- 
tain agencies  to  be  described  in  a  later  chapter  and  known  as 
metamorphism  may  become  densely  hard  and  crystalline,  and  is 
then  called  marble.  The  discussion  of  the  stratified  rocks  which 
have  been  subjected  to  metamorphism  is,  however,  deferred  to  that 
chapter  in  which  this  subject  is  treated;  here  only  those  cases  are 
considered  where  the  sediments  have  been  consolidated  by  pressure 
and  cementation,  as  previously  described.  The  chief  sediments  and 
the  rocks  they  yield  are,  then,  as  follows: 


Sediments 

Compacted  strata,  as 
rocks 

Gravel.  .  .  . 

Conglomerate 

Sand  
Silt  and  clay  (mud)  .... 
Lime  deposits. 

Sandstone 
Shale 
Limestone 

Gradations  of  Rocks.  —  It  must  not  be  imagined  that  the  differ- 
ent kinds  of  rocks  mentioned  above  are  always  sharply  defined  from 


SEDIMENTARY   ROCKS  281 

one  another  as  clear  distinct  types.  This  is  very  far  from  being  the 
case.  Just  as  muds  grade  through  sand  into  gravel,  and  pure  lime 
deposits  into  muddy  ones,  so  may  the  various  rocks  formed  from 
them  grade  into  one  another. 

At  this  point  it  may  be  well  to  explain  the  usage  of  certain  terms  frequently 
employed  in  connection  with  sediments  and  stratified  rocks.  Of  the  finer  de- 
posits, or  muds,  clay  is  the  most  important  representative;  the  word  clay  is 
of  Anglo-Saxon  origin,  its  adjective  is  clayey,  the  corresponding  word 
derived  from  the  Latin  is  argillaceous  (from  the  Greek  argillos,  clay). 
Similarly  the  adjective  sandy  has  its  Latin  equivalent  in  arenaceous  (from 
arena,  sand  —  the  place  where  gladiatorial  combats  took  place  was  so  called 
because  covered  with  it).  The  adjective  limy,  little  used,  has  its  counter- 
part in  calcareous  (from  the  Latin  calx,  lime,  limestone).  These  adjectives, 
argillaceous,  arenaceous,  and  calcareous,  are  constantly  used  with  reference 
to  the  sediments  to  which  they  belong  and  the  rocks  composed  of  them. 

The  gradations  of  the  various  kinds  of  stratified  rocks  into  one 
another  may  be  illustrated  in  the  following  diagram: 

Pure  Limestone 
(Calcareous) 


Argillaceous  Limestone  /  \  Arenaceous  Limestone 

(Shaly)  /  Limestones  V          (Sandy) 


Limestones 

5  per-cent 

of  the  whole 


Shales 

80  per-cent  of 
the  whole 


Sandstones 

15  per-cent  of 

the  whole 


Calcareous  Shale /  \  /  \CalcareousSandstone 


Pure  Shale  Z ™e  wnoie L_^n^__Apure  Sandstone 

(Clay,  Mud,  etc.)  Arenaceous  Argillaceous  (Sand,  Arenaceous) 

(Argillaceous)  Shale  Sandstone 

We  are,  therefore,  accustomed  to  speak  of  calcareous  sandstones, 
shaly  limestones,  etc.,  as  indicated  in  the  above  diagram.  The  per- 
centages in  it  give  the  relative  estimated  proportions,  in  each  kind, 
of  the  total  volume  of  all  sedimentary  rock;  thus  shales  are  sixteen 
times  as  abundant  as  limestones. 

The  characteristic  features  of  those  stratified  rocks  which  are  of 
greatest  importance  are  the  following: 

Conglomerate.  —  The  rock  consists  of  pebbles  or  fragments  held 
together  by  a  base  or  cement  of  some  kind.  The  pebbles,  which 
compose  the  gravels  from  which  these  rocks  are  formed,  are  gen- 
erally rounded.  Quartz  is  the  most  common  substance  constituting 
them,  but  they  may  consist  of  pieces  of  definite  rock,  such  as  granite, 


TEXT-BOOK   OF   GEOLOGY 

basalt,  etc.  Such  pebbles  may  vary  greatly  in  size,  from  a  fraction 
of  an  inch  to  a  couple  of  feet,  or  more,  in  diameter.  The  appear- 
ance of  a  conglomerate  is  shown  in  Fig.  209.  When  the  contrast  of 
pebbles  and  cement  is  clearly  marked,  such  rocks  are  sometimes 
called  pudding-stone.  When  the  rock  is  composed  of  angular  frag- 
ments, which  is  sometimes  the  case  where  the  material  has  suffered 


Fig.  209.  —  Conglomerate;   the  pebbles  in  this  case  are  about  the  size  of  an  egg,  and 
composed  of  several  different  kinds  of  rocks. 

no  transport,  or  only  a  short  one,  it  is  called  breccia.  Some  con- 
glomerates show  by  their  characters,  especially  by  the  grooved  and 
striated  surfaces  of  the  pebbles  and  the  facets  ground  upon  them, 
that  they  are  ancient  bowlder-clays,  or  tills,  the  morainal  deposits  of 
glaciers  and  ice-caps  of  past  geologic  ages.  See  page  144. 

Sandstones.  —  These  are  usually  quite  even  in  grain  and  vary 
from  friable  to  firm,  according  to  the  strength  of  the  cement.  The 
grains  are  composed  wholly,  or  mostly,  of  quartz.  In  the  red  and 
brown  varieties  the  cement  is  mainly  oxide  of  iron;  in  the  white, 
buff,  gray,  or  pale  brown,  carbonate  of  lime;  the  question  of  their 


SEDIMENTARY   ROCKS  283 

solubility  has  been  already  discussed,  page  162.  Sandstones  are 
generally  very  porous  rocks;  30  per  cent  of  their  volume  may, 
indeed,  consist  of  interspaces  between  the  grains;  they  are,  there- 
fore, favorable  strata  in  which  to  find  artesian  water.  See  page  158. 

Arkose  is  a  variety  of  sandstone  which  contains  much  unaltered  feldspar. 
Its  occurrence  indicates  that  the  component  material  has  not  been  long  ex- 
posed to  weathering,  and  has,  therefore,  probably  not  been  transported  great 
distances.  It  is  thus  more  likely  to  be  of  continental,  than  of  marine,  origin, 
and  to  be  produced  by  the  breaking  down  of  rock  in  lands  with  cold,  arid 
climates,  rather  than  in  those  with  warm  humid  ones  where  rock  decay  is 
rapid. 

Graywacke.  —  This  is  a  sandstone-like  rock  of  prevailing  gray  color,  com- 
posed of  grains  of  various  minerals  and  tiny  fragments  of  other  rocks.  It  is 
really  a  very  fine-grained  conglomerate,  or  not  infrequently  passes  into  one 
by  increase  in  the  size  of  the  particles.  It  may  also  be  of  continental  origin. 

Shale.  —  This  name  is  given  to  compacted  muds  and  clays  which 
possess  a  more  or  less  thinly  laminated,  or  fissile,  structure,  along 
which  they  may  be  rather  easily  cleaved.  This  parting  is  parallel 
to  the  bedding  and  is  the  result  of  natural  stratification.  Where 
shale  beds  have  been  subjected  to  folding  and  pressure,  by  crump- 
ling of  the  crust,  they  become  generally  harder  and  assume  a  slaty 
cleavage  which  is  distinct  from  stratification;  the  rocks  are  then 
slates,  not  shales,  and  will  be  discussed  under  metamorphic  kinds 
in  a  following  chapter.  Shales  are  soft  rocks,  can  be  cut  with  a 
knife,  and  are  apt  to  be  brittle,  and  to  readily  break  up  into  small 
chips.  They  show  a  great  variety  of  colors  from  light  to  dark;  in 
the  latter  case  organic  matter  is  present.  Like  clay,  of  which  to  a 
greater  or  lesser  degree  they  are  composed,  they  yield  a  character- 
istic odor  when  breathed  upon.  Unlike  sandstone,  they  tend  to  be 
impermeable  to  water. 

Limestones.  —  The  chief  varieties  of  the  carbonate  rocks  are 
limestone  proper  (essentially  carbonate  of  lime,  CaCO3) ;  dolomite, 
in  which  more  or  less  of  the  lime  has  been  replaced  by  magnesia  to 
form  the  dolomite  molecule  (MgCa)C03,  and  chalk.  These,  and 
some  of  their  sub-varieties,  such  as  coquina,  have  been  described 
under  organic  life,  page  190  and  following,  where  their  origin  is 
treated.  Limestones  are  usually  bluish  in  color,  or  vary  from  pale 
gray  to  black,  dependent  on  the  amount  of  organic  matter  they 
contain;  sometimes  they  are  yellowish  or  brown.  They  are  gen- 
erally very  dense,  compact  rocks,  in  some  cases  very  homogeneous, 
in  others  more  or  less  filled  with  shells,  or  other  fossil  forms.  They 
can  usually  be  distinguished  from  other  rocks,  which  they  may 


284  TEXT-BOOK  OF  GEOLOGY 

resemble,  by  the  readiness  with  which  they  can  be  scratched,  or 
cut,  and  by  their  effervescing  when  treated  with  acid. 

Some  Minor  Sedimentary  Rocks.  —  In  this  connection  brief  mention  may 
be  properly  made  in  this  place  of  some  minor  varieties  of  sedimentary  rocks. 
They  are  coal,  iron-ore,  rock-salt,  and  gypsum.  The  reason  that  they  are 
spoken  of  as  minor  deposits  is,  that,  although  of  vast  importance  from  the 
human  standpoint,  geologically  they  occur  in  volumes  so  limited,  as  com- 
pared with  the  enormous  bulk  of  the  shales,  sandstones,  and  limestones,  as 
to  be  of  little  importance,  when  considered  merely  as  rock  masses. 

a.  Coal.  —  The  formation  of  peat  and  its  relation  to  coal  has  been  already 
mentioned,  pages  175-180.  When  peat  changes  into  coal  the  process  is  a  grad- 
ual one;  the  organic  material  is  buried  in  layers  of  clays  and  sands,  and  as 
these  change  into  shales  and  sandstones,  it  is  turned  into  coal,  by  loss  of 
volatile  matter  and  pressure.    This  takes  place  in  various  stages;  first,  brown 
coal  or  lignite  is  formed,   a  rather  soft,  lusterless  brownish   material  with 
about  60  per  cent  of  carbon;  in  a  more  advanced  stage  this  becomes  soft  or 
bituminous  coal,  a  compact,  black,  brittle  rock  with  75-90  per  cent  carbon; 
under  proper  geologic  conditions,  soft  coal  may  change  to  anthracite,  a  dense, 
black,  shining  rock,  with  80-95  per  cent  of  carbon.    Further  details  regarding 
the  nature  of  this  change,  and  the  formation  of  coal,  will  be  found  in  the 
second  part  of  this  work  in  connection  with  the  occurrence  of  coal. 

b.  Iron-ore.  — Beds  of  iron-ore  varying  from  a  few  inches  to  many  feet  in 
thickness,  are  often  found  in  association  with  stratified  rocks,  and  in  varying 
degrees  of  purity.    It  is  either  limonite,  the  hydrated  oxide  of  iron,  or  siderite, 
ferrous   carbonate     One   mode   of   formation   has   already   been   treated   of, 
pages  172  and  181.    Other  information  respecting  these  ores  will  be  found  on 
page  421,  as  minerals  in  the  Appendix,  and  concerning  their  occurrence  in  the 
second  part  of  the  book. 

c.  Rock-salt.  —  Beds  of  salt  are  also  found  associated  with  stratified  rocks, 
especially  with  clays  and  shales;  they  are  rather  limited  in  area,  but  some- 
times of  enormous  thickness.    The  manner  of  salt  formation  has  been  con- 
sidered on  pages  84-89. 

d.  Gypsum.  —  This  substance,  the  hydrated  sulphate  of  lime,  is  produced 
under  arid  conditions,  like  salt,  which  indeed  it  is  very  apt  to  accompany, 
though  it  also  occurs  independently.    It  is  found  mostly  in  thick  lenses,  or 
beds  of  limited  area.    Its  mineral  characters  are  stated  in  the  Appendix,  and 
occurrences  are  given  in  Part  II. 

Characteristic  Features  of  Sedimentary  Rocks 

In  addition  to  the  ordinary  stratification,  which  these  rocks 
exhibit  as  a  proof  of  their  mode  of  origin,  they  also  possess  other 
features,  some  of  them  minor  ones,  it  is  true,  but  none  the  less 
of  significance,  which  enable  us  to  determine  the  places  where 
the  sediments  were  deposited  and  the  conditions  under  which  the 
deposition  took  place,  and  to  thus  throw  light  upon  the  geological 
history  which  they  record.  Some  of  the  more  important  of  these 
features  may  be  tabulated  as  follows: 


SEDIMENTARY   ROCKS  285 

Fossil  Remains  of  Former  Life.       Ripple-  and  Rill-marks. 
Foot-prints  and  Other  Tracks          Cross-bedding. 

of  Animals.  Conglomerate  Structure. 

Rain-drop  Impressions.  Oolitic  Structure. 

Mud-cracks. 

Fossils.  —  It  is  a  common  and  well-known  fact  that  the  strati- 
fied rocks  contain  in  variable  amount  the  remains  of  animals  and 
plants  inhabiting  the  earth  in  former  times.  Sometimes  these  con- 
sist simply  of  the  impressions  of  the  organisms  in  the  rock,  some- 
times in  the  actual  preservation  of  their  hard  parts,  such  as  bones 
and  shells,  and  sometimes  in  the  complete  preserval  of  the  whole 
organic  structure  by  its  entire  change  into  stone  (petrifaction), 
particle  by  particle,  as  the  organic  matter  decayed,  or  was  removed. 
Great  diversity  in  the  fossils  of  the  rocks  is  found  in  several  ways, 
and  for  obvious  reasons.  Thus  they  may  vary  according  to  the 
kind  of  rock,  or,  as  it  is  said,  change  according  to  the  rock  facies; 
the  kinds  of  animals  that  live  in  muds  differ  from  those  inhabiting 
sands,  to  a  certain  extent,  and  thus  sandstones  are  liable  to  contain 
different  fossils  from  shales.  Fishes,  which  are  free-swimming  ani- 
mals inhabiting  both  salt  and  fresh  water,  might  furnish  fossils, in 
nearly  all  the  different  kinds  of  deposits,  either  continental  or 
marine,  while  the  bones  of  land  animals  would  be  expected  in  the 
former  rather  than  in  the  latter,  especially  in  sandstones.  Also  the 
fossils  found  in  the  earlier  rocks  are  very  different  kinds  from 
those  of  the  strata  formed  in  later  geological  periods,  and  we  learn 
by  attentive  study  of  this  fact  that  there  has  been  a.  constant  evo- 
lution of  life  upon  the  earth,  from  very  simple  to  more  and  more 
complex  types  of  organic  structures.  And,  finally,  just  as  we  know 
that  the  associated  animals  and  plants  of  one  part  of  the  earth 
(its  fauna  and  flora)  differ  from  those  of  another  part,  so  do  the 
fossils  of  one  region  differ  from  those  of  another  region.  The  fur- 
ther back  in  the  rocks  we  go,  however,  the  less  marked  do  we  find 
this  difference,  since  the  nearer  do  we  approach  to  the  less  modi- 
fied and  simpler  types  of  marine  life. 

All  these  are  facts  of  the  highest  importance,  and  the  manner  in 
which  they  are  used  in  deciphering  the  past  history  of  the  earth 
and  its  inhabitants  will  be  set  forth  in  detail  in  the  second  part  of 
this  book. 

Foot-prints,  and  Rain-drop  Impressions.  —  These  are  features 
not  infrequently  found  in  stratified  rocks,  and  the  conditions  under 
which  they  are  formed  appear  to  be  as  follows:  The  tracks  of  large 


286  TEXT-BOOK   OF  GEOLOGY 

vertebrate  animals  made  in  soft  muds  and  clays  of  the  land  sur- 
faces of  to-day,  as  in  the  time  of  spring  rains  and  floods,  become, 
laier  in  the  season,  especially  in  arid  and  semi-arid  countries,  baked 
to  an  almost  brick-like  hardness,  and  may  endure  for  several  years 
before  they  are  effaced.  The  same  is  true,  in  lesser  degree,  of  the 
tracks  of  birds  and  small  mammals,  and  the  pits  made  by  the  rain- 
drops of  a  passing  shower.  This  could  hardly  occur  where  the  de- 
posit was  kept  soft  and  frequently  washed,  as  in  humid  regions,  with 
much  rainfall,  or  on  mud-flats  constantly  subjected  to  the  action  of 
the  tide.  But,  on  the  mud-flats  of  river  plains  and  deltas,  and  on  the 
shores  of  interior  basins  of  arid  or  semi-arid  regions,  we  can  imagine 
such  impressions  formed,  hardened,  and  then  covered  by  deposits 
blown  by  the  wind,  or  swept  by  the  waters  of  the  next  flood-time, 
and  thus  preserved.  They  are  thus  essentially  features  of  conti- 
nental deposits,  and  will  only  rarely  occur  in  those  of  the  littoral 
regions  of  the  sea,  where,  possibly  in  the  mud-flats  at  the  heads  of 
estuaries,  conditions  favorable  for  their  production,  such  as  unusu- 
ally high  tides,  might  sometimes  be  present.  Since  impressions  in 
sand  generally  do  not  retain  their  shape  or  are  quickly  effaced,  it  is 
evident  that  these  prints  must  be  usually  made  in  mud,  or  muddy 
sands,  to  be  permanent,  and  this  means  that  those  of  former  ages 
will  be  found  chiefly  in  shale,  or  very  shaly  sandstones,  as  this  is 
the  rock-form  of  compacted  muds  and  clays.  Examples  of  foot- 
print and  rain-drop  impressions  in  shale  may  be  seen  in  the 
second  part  of  this  book.  It  is  of  course  clear  that  such  impressions 
could  not  be  found  in  true  marine  deposits. 

Mud-cracks.  —  What  has  been  stated  above  of  foot-prints  is 
quite  as  true  of  mud-cracks.  Soft  muds  left  exposed  by  the  reces- 
sion of  high  water  dry  and  crack  into  polygonal  forms,  as  illus- 
trated in  Fig.  210.  Further  exposure  to  the  sun  bakes  and  hardens 
the  blocks.  During  the  dry  season  these  are  often  covered  and 
preserved  by  wind-blown  silt  or  sand.  In  other  instances,  at  the 
next  period  of  high  water,  these  cracks  will  be  filled  with  the  coarser 
sediment  first  deposited,  and  the  whole  record  buried  and  pre- 
served by  succeeding  deposits.  After  the  whole  series  of  deposits 
has  been  hardened  into  rock,  usually  with  subsidence  beneath  the 
sea  and  subsequent  emergence,  the  layers  of  shale,  which  the  muds 
and  clays  form,  may  be  exposed  by  erosion,  quarrying,  etc.,  and  will 
then  exhibit  these  mud-cracks.  It  may  happen  that  in  taking  up 
the  rock-layers  the  soft  shale  beds  will  break  away,  leaving  the 
filling  of  the  mud-cracks  projecting  from  the  lower  surface  of  the 
overlying  layer,  thus  furnishing  a  natural  cast  of  the  cracks.  As 


SEDIMENTARY   ROCKS 


287 


in  the  case  of  foot-prints,  it  is  obvious  that  the  most  favorable  places 
for  their  occurrence  will  be  on  the  flood  plains  of  rivers,  on  the 
wide  flat  shores  of  shallow  interior  lakes,  and  less  prominently  at  the 
upper  margins  of  shallow  estuaries  of  the  sea,  and  especially  under 
arid  or  semi-arid  conditions  of  climate,  where  time  for  sun-baking 
and  hardening  occurs  between  periods  of  flood-water.  They  are 
thus  characteristic  features  of  continental  deposits,  minor  and  re- 
stricted ones  of  littoral  deposits,  and  in  marine  deposits  will  be  gen- 
erally wanting. 


Fig.  210.  —  Mud-cracks,  delta  of  the  Colorado  River.     G.  K.  Gilbert,  U.  S.  Geol.  Surv. 

Under  certain  conditions,  however,  which  have  sometimes  occurred  in  the 
geologic  past,  the  recession  of  stretches  of  very  shallow  sea-waters,  through 
other  causes  than  the  tides  or  infilling  by  sediments,  has  resulted  in  the  drying 
and  cracking  of  marine  sediments,  especially  of  limy  oozes,  but  it  is  to  be 
noted  that  the  structure  still  signifies  a  continental  origin.  Thus  mud- 
cracked  limestones  are  not  infrequently  observed. 

Submarine  Tracks.  —  Other  markings  and  structures  are  observed  in  the 
stratified  rocks,  which  by  study  and  comparison  with  similar  features  found  in 
modern  sediments,  are  referred  to  the  tracks  made  by  marine  animals,  such 
as  crabs  walking  on  the  bottom,  to  the  trails  left  by  worms  crawling,  and 
to  the  filling  of  burrows  dug  by  such  animals  in  the  deposit.  They  can  occur 
only  when  the  material  has  a  tenacity  adequate  to  retain  the  impressions  until 


288 


TEXT-BOOK  OF  GEOLOGY 


covered  by  the  next  sedimentation;  it  must,  therefore,  be  neither  too  soft  nor 
too  crumbly.  Sand  mixed  with  considerable  clay,  giving  rise  to  shaly  sand- 
stone, is  the  most  natural  medium.  Such  impressions  on  a  beach,  subjected 
to  the  ebb  and  flow  of  the  tide  and  the  action  of  waves,  could,  in  general,  not 
be  permanent;  hence  their  occurrence  is  indicative  of  marine  deposits,  formed 
especially  in  shallow  water. 

Ripple-marks.  —  If   one   observes   a   sandy   bottom  in  shallow 
water,  it  will  be  frequently  noticed  that  the  sand  has  been  thrown  up 


Fig.  211.  —  Ripple-marked  sandstone. 

into  a  series  of  small  parallel  ridges  by  the  action  of  the  waves. 
These  are  known  as  ripple-marks.  While  they  may  also  be  formed 
in  sand  on  land  by  the  action  of  the  wind,  as  on  sand-dunes,  see 
Fig.  5,  they  are  also  characteristic  of  wave  action  in  shallow 
water.  They  are  not  directly  heaped  by  the  waves,  but  are  caused 
by  the  oscillatory  motion  which  these  give  to  the  water,  and,  while 
generally  formed  in  depths  of  less  than  100  feet,  they  may,  it  is 
stated,  corrugate  the  bottom  in  very  fine  sands  up  to  600  feet  after 
heavy  storms.  Such  furrowings  may  be  preserved  by  later  deposits, 
and,  when  consolidated  into  stone,  give  the  rock  surfaces  a  charac- 
teristic appearance  when  split  or  exposed  to  erosion,  see  Fig.  211. 

In  deep  water  the  ripple-marks  appear  to  be  symmetrical,  closely  spaced, 
and  due  to  wave  action.  They  have  been  termed  oscillation-ripples.  In  shal- 
lower water  they  may  be  caused  by  currents,  and  are  not  symmetrical;  the 


SEDIMENTARY   ROCKS 


289 


gentle  slope  is  on  the  side  of  current  arrival;  these  are  current-ripples.  In 
shallow  water  there  may  also  be  produced  giant-ripples,  due  to  tidal  action, 
with  crests  possibly  6-8  feet  high  and  60-100  feet  apart;  see  also  remarks  on 
heaping  by  stream  traction,  page  42. 

Ripple-marks,  in  their  size  and  spacing,  depend  in  some  measure  on  the 
coarseness  of  the  grains,  and  on  the  depth,  being  more  closely  spaced  with 
fine  material  and  in  deeper  water,  and  to  a  less  degree  on  the  intensity  of 
winds  and  waves. 


Fig.  212.  —  Cross-bedding  in  sandstones.  The  large  scale  on  which  it  occurs  indi- 
cates a  probable  seolian  (dune)  origin.  Walnut  Canyon,  Ariz.  J.  K.  Killers, 
U.  S.  Geol.  Surv. 

Wave-marks,  occasionally  seen  on  the  surface  of  strata,  are  the 
curved  lines  of  material  washed  up  on  the  beach  at  the  inshore 
edges  of  waves,  while  rill-marks  represent  the  little  diverging  chan- 
nels cut  by  the  returning  under-tow  in  passing  over  pebbles  and 
other  obstacles  in  the  sand,  or  by  rain  water  running  over  exposed 
flats.  These  are  indicative  of  littoral  and  continental  deposits, 
although  it  is  possible  that  somewhat  similar  markings  may  also  be 
aeolian  in  origin. 

Cross-bedding,  or  Oblique  Lamination.  —  It  is  frequently  ob- 
served in  strata  composed  of  the  coarser  detritus,  such  as  conglom- 
erates and  sandstones,  that  the  laminae  of  particular  beds,  instead  of 
being  parallel  to  the  general  planes  of  stratification  of  the  series,  are 
inclined  to  them,  often  at  considerable  angles,  and  perhaps  curved  as 
well,  as  shown  in  the  illustration,  Fig.  212.  This  structure  is  known 


290  TEXT-BOOK   OF   GEOLOGY 

as  cross-bedding,  or  oblique  lamination.  It  usually  indicates  rapid 
deposit  in  shoal  water  by  quick  and  shifting  currents,  and  is  liable  to 
occur  in  the  foreset  beds  (page  277)  of  deltas,  bars,  spits  and  barrier 
beaches,  where  the  material  is  dropped  down  the  forward  slope  of 
an  advancing  deposit.  It  may  happen  in  rivers,  lakes  and  the 
shallow  waters  of  the  sea,  and  thus  be  found  in  continental,  littoral, 
and  marine  deposits.  It  is  also  the  characteristic  structure  of  wind- 
built  sand-dunes  where  the  material  is  rapidly  shifted  about  and 


Fig.  213.  —  Concretions  from  clay  beds,  Long  Island. 

commonly  deposited  on  inclined  surfaces.  It  may  thus  be  aeolian  in 
origin  and,  therefore,  again  found  in  continental  deposits.  The 
cross-bedding  of  certain  sandstones  of  the  Colorado  region,  and  of 
the  chalky  limestones  of  Bermuda  has  been  considered  to  have  this 
seolian  origin. 

Concretions,  and  Concretionary  Structure 

Concretions.  —  Stratified  rocks  in  many  places  contain  numer- 
ous inclusions  of  a  nature  different  from  that  of  the  material  enclos- 
ing them.  These  inclusions  are  apt  to  be  rounded,  and  nodular  in 
form;  some  occurrences  are  quite  spherical,  others  flattened,  ovate, 
elongated,  ring-shaped,  or  compound  and  exhibiting  odd  and  fan- 
tastic shapes.  They  may  be  a  fraction  of  an  inch  in  diameter  or 


SEDIMENTARY    ROCKS 


291 


many  feet.  The  shapes  of  some  are  shown  in  Fig.  213  and  the  mode 
of  occurrence  in  Fig.  214.  They  are  often  arranged  in  parallel 
layers.  On  breaking  them  open  it  is  often  found  that  the  globular 
mass  consists  of  matter  aggregated  about  some  object  as  a  nucleus. 
Masses  of  this  kind  are  known  as  concretions.  In  composition  they 
are  different  from  that  of  the  main  rock  mass  in  which  they  lie,  and 
are  formed  from  one  of  its  minor  constituents;  thus  in  chalk  and 


Fig.  214.  —  Concretions  in  clay,  Los  Angeles,  Cal.     R.  Arnold,  U.  S.  Geol.  Surv. 

limestone  they  are  composed  of  silica ;  in  sandstone,  of  iron  oxide  or 
carbonate  of  lime ;  in  shale,  of  carbonate  of  lime  or  sulphide  of  iron. 
While  some  are  very  pure,  they  often  contain  large  amounts  of  the 
rock-material,  and  in  some  cases  the  planes  of  stratification  can  be 
seen  passing  through  them.  Their  origin  appears  to  be  due  to 
material  in  the  rock  having  gone  into  solution,  and  then  for  some 
reason  having  been  steadily  redeposited  around  certain  centers  as 
nuclei,  thus  building  up  the  concretions. 

Sometimes  the  bodies  of  animals  or  leaves  of  plants  decaying  in  the  sand  or 
mud  appear  to  have  been  the  determinant  cause  of  the  formation  of  concretions, 
and  to  have  served  as  the  nuclei  about  which  they  collected.  On  splitting  open 
such  concretions,  remarkable  fossil  imprints  of  ferns,  insects,  and  marine  animals 
like  shrimps,  fishes,  etc.,  may  be  obtained.  Or  sometimes,  when  they  attain 
huge  dimensions,  the  shells  and  bones  of  large  animals  may  be  found  en- 
closed in  them.  In  other  cases  some  inorganic  substance,  such  as  a  grain  of 
sand,  may  have  formed  the  nucleus,  while  in  others  no  definite  nucleus  can  be 
found.  In  some  concretions  cracks  occur  in  which  mineral  matter  of  another 


292  TEXT-BOOK  OF  GEOLOGY 

kind  has  been  deposited,  seaming  the  surface  with  a  polygonal,  network  of 
veins;  these  are  known  as  septaria.  Iron-oxide  concretions  are  not  infre- 
quently hollow,  and  more  or  less  filled  with  sand.  See  also  Fig.  18. 

Flint  and  Chert.  —  Flint  is  a  dark  gray  to  black,  very  hard  and  compact  sub- 
stance occurring  in  irregular  nodules,  or  concretions,  in  chalk.  It  is  composed 
of  silica,  SiO2,  with  a  little  chemically  combined  water.  An  impure  flint, 
occurring  in  a  similar  way  in  limestones,  is  known  as  chert;  it  is  sometimes 
seen  in  parallel  layers  and  lenses  in  the  rock.  The  silica  composing  these 
substances  appears  in  some  cases  to  have  been  derived  from  the  hard  parts 
of  certain  organisms  living  in  the  sea-water,  such  as  sponges,  radiolarians, 


Fig.  215.  —  Pisolite,  showing  concretionary  structure.     Natural  size.     Bohemia. 

teeth  of  worms,  etc.,  which  have  gone  into  solution  and  been  redeposited. 
Flint  is  a  substance  of  considerable  interest  because,  on  account  of  its  hard- 
ness, homogeneity,  and  lack  of  cleavage,  it  has  been  used  extensively  from 
prehistoric  times  down  to  the  present  by  primitive  peoples  in  the  manufac- 
ture of  implements  and  weapons,  and  for  its  employment  in  striking  fire. 
Until  a  comparatively  recent  time,  when  percussion  caps  were  invented,  great 
battles  and  the  fate  of  nations  were  decided  by  flint-lock  guns.  It  has  been 
observed  that,  in  the  weathering  and  decay  of  certain  Paleozoic  limestone 
formations  in  the  Southern  States,  nodules  and  plates  of  flint  are  formed, 
which  may  accumulate  thickly  on  the  surface.  The  chemistry  of  this  process 
is  not  well  understood.  Further  details  with  respect  to  flint  will  be  found 
in  the  Appendix. 

In  some  places  considerable  masses  of  flint-like  rocks  occur  on  a  scale  which 
appears  much  too  great  for  them  to  be  explained  by  the  origin  mentioned 
above.  The  varieties  are  sometimes  called  jaspilite  (Lake  Superior  region), 
sometimes  novaculite  (Arkansas),  and  sometimes  other  names  are  given  to 
them.  In  some  instances  they  are  believed  to  have  been  formed  from  silica 
chemically  precipitated  from  solution;  in  others  their  origin  is  uncertain. 


SEDIMENTARY   ROCKS  293 

Concretionary  Structure,  Oolite.  —  Concretions  may  become  so 
numerous  in  a  rock  stratum  that  its  entire  mass  is  composed  of 
them,  giving  rise  to  concretionary  structure.  While  this  has  been 
observed  in  sandstones  and  in  iron-ore  deposits,  as  in  the  Clinton 
ores  of  eastern  North  America,  it  is  mostly  seen  in  certain  lime- 
stone rocks.  The  concretions  are  generally  minute,  like  small  shot, 
and  the  rock  in  appearance  resembles  fish-roe,  and  hence  is 
called  oolite  (egg-stone).  A  variety  with  larger  concretions  (Fig. 
215)  is  known  as  pisolite  (pea-stone).  The  oolite  structure  is  not 
rare ;  indeed  it  is  much  more  common  in  limestones  than  is  generally 
realized. 

On  the  shores  of  Great  Salt  Lake  at  the  present  time  the  sand  is  observed 
to  consist  of  minute  spherical  concretions,  like  fine  bird-shot,  of  carbonate  of 
lime  deposited  from  the  lake  waters.  Similar  sands  are  forming  about  some 
coral  islands;  such  sands  if  compacted  and  cemented  would  form  oolite,  and 
in  some  places  this  formation  of  the  rock  is  now  taking  place,  thus  throwing 
light  on  its  origin  in  the  past. 


Dimensions  of  Beds;  Overlap;  Relative  Age 

Area  and  Form  of  Beds.  —  It  is  theoretically  conceivable  that 
if  all  the  lands  were  beneath  the  sea  a  world-wide  stratum  consisting 
of  deposits  by  marine  organisms  might  form,  and  on  re-elevation  of 
the  lands  be  everywhere  found  upon  them.  No  instance  of  this 
kind  is  known  and  the  teachings  of  geological  history,  as  shown 
later,  inform  us  that  it  has  not  taken  place.  On  the  other  hand, 
beds  of  mechanical  sediments,  such  as  sandstones  and  shales,  imply 
land  surfaces  from  which  they  are  derived,  and  basins  in  which  they 
are  laid  down;  it  is  obvious  that  their  areal  extent  must  be  limited 
by  the  borders  of  the  basins,  next  to  which  they  must  thin  out  and 
disappear.  In  geometric  form,  then,  a  bed  must  be  lens-like,  lentic- 
ular, thicker  near  one  border  than  on  the  other,  and  commonly  ex- 
tremely flattened;  in  ground  plan  it  need  not  be  circular,  but  is 
usually  much  elongated  and  irregular.  As  a  general,  but  not  in- 
variable, rule  it  is  found  that  the  coarser  the  material  composing  a 
stratum  is,  the  smaller  the  area  which  it  covers ;  thus  conglomerates 
may  die  out  rapidly  in  a  few  miles,  or  even  less,  and  coarse  sand- 
stones may  have  the  same  inconstancy,  while  on  the  other  hand, 
beds  of  shale  and  limestone  have  been  observed  to  cover  thousands 
of  square  miles.  Also,  dependent  upon  the  law  of  sedimentation, 
see  page  271,  it  is  found  that  beds  are  thickest  and  coarsest  near  the 
source  of  supply,  and  thin  out  and  become  finer  in  texture  as  one 


294 


TEXT-BOOK   OF   GEOLOGY 


recedes  from  it.  Thus,  coarse  sandstones  are  observed  to  grade  into 
finer,  and  these  into  shales.  It  is  sometimes  stated  that  beds  of 
stratified  rock,  if  not  changed  from  their  original  position,  are  par- 
allel and  horizontal  when  elevated  to  form  land-surfaces.  From 
what  has  been  mentioned  above  it  is  clear  that  this  is  not  absolutely 
true;  yet  the  scale  on  which  the  beds  form  unsymmetrical  lenses  is 
usually  so  large  that,  generally,  in  the  exposures,  even  over  long  dis- 
tances, they  may  appear  to  rigorously  follow  this  rule.  See  Fig.  205. 

That  the  strata  are  not  always  laid  down  horizontally  may  be  readily  seen 
where  cross-bedding  (previously  mentioned)  occurs,  and  especially  in  the  fore- 
set  beds  of  small  deltas,  bars,  etc.  The  inclination  of  such  beds  may  be  as 
much  as  10°  to  the  horizontal.  Since  the  great  extended  beds  of  sediment 
have  a  one-sided  lenticular  form  with  the  thickest  edge  next  to  the  land,  as 
they  are  piled  on  one  another,  the  planes  of  stratification  must  have  a  gentle 
inclination  seaward.  But  the  scale  on  which  this  occurs  is  often  so  great  that, 
as  mentioned  above,  we  may  not  perceive  it. 


Fig.  216.  —  Diagram  to  illustrate  overlap.  On  a  sinking  land,  as  the  water,  WW, 
continually  moves  inland  on  the  shore,  Sh,  the  successive  beds  of  sediment  a,  b,  c, 
are  laid  down  more  and  more  to  the  left,  the  edges  of  successive  upper  and  newei 
beds  overlapping  those  of  the  older  and  lower  ones.  On  a  rising  land  this  relation 
would  be  reversed. 

Overlap.  —  It  is  often  observed  in  a  series  of  strata  that  the 
edges  of  some  beds  extend  for  distances  beyond  those  of  beds  below 
them,  while  the  lower  ones  thin  out  and  disappear.  This  relation 
is  known  as  overlap,  and  the  manner  in  which  it  originates  may  be 
understood  by  reference  to  the  diagram,  Fig.  216.  This  gradual 
advance  of  the  sediments  upon  one  another  (or  their  retreat)  is  of 
interest,  because  it  marks  the  position  of  ancient  shore-lines,  and  in- 
dicates a  sinking  or  rising  of  the  land  with  shifting  of  the  shore-lines, 
or,  possibly,  the  sea  itself  may  rise  or  sink.  Overlap  may  also  occur 
on  a  small  scale  in  the  filling  by  deposit  of  a  basin  with  sloping  sides. 
It  must  not  be  confused  with  nonconformity. 

Thickness  of  Sediments.  —  It  has  been  customary  to  ascribe 
great  thickness  to  accumulated  beds  of  sediments,  or  in  other  terms 
to  stratified  rocks,  in  certain  places,  especially  in  mountain  ranges. 
It  is  very  common  to  find  them  measured  by  thousands  of  feet; 
5,000,  10,000  and  15,000  are  not  uncommon,  and  in  some  regions 
even  greater  thicknesses  have  been  ascribed  to  them;  the  strata 
which  now  compose  the  Appalachian  Mountains  are  held  to  be 


SEDIMENTARY   ROCKS  295 

30,000  feet  thick  in  Pennsylvania,  while  those  of  the  Alps  have  been 
placed  at  50,000.  It  is  obvious  that  if  we  accept  such  great  thick- 
nesses as  correct,  since  the  great  bulk  of  the  land-derived  sediments 
is  deposited  near  shore,  it  must  follow  that  subsidence  of  the  sea- 
floor  also  occurred  to  permit  of  their  accumulation.  For  close  to  the 
shore  we  do  not  find  water  of  any  such  depth  —  30,000  feet,  indeed, 
marking  the  deepest  parts  of  the  ocean.  And  even  though  these 


Fig.  217.  —  To  illustrate  apparent,  as  compared  with  real,  thickness  of  strata.  The 
figures  show  a  delta  deposit  composed  of  inclined  foreset  beds.  The  real  thick- 
ness of  the  formation  is  ed.  The  apparent  thickness  cb,  obtained  by  multiplying 
the  distance  ab,  the  exposed  edges  of  the  uplifted  formation,  by  the  sine  of  the 
angle  of  inclination,  cab,  is  evidently  much  too  great.  Modified  from  Chamber- 
lin  and  Salisbury. 

maximum  thicknesses  assumed  may  contain,  as  they  usually  do, 
marine  deposits  of  carbonate  of  lime  in  large  amount,  in  addition  to 
the  terrigenous  material,  subsidence  would  be  just  as  necessary. 
But,  while  undoubtedly  subsidence  and  the  accumulation  of  sedi- 
ments to  great  thicknesses,  many  thousands  of  feet,  have  occurred, 
it  is  at  least  questionable  whether  in  the  extreme  cases,  such  as  those 
mentioned,  all  the  different  data  necessary  for  an  accurate  result  in 
measuring  the  thickness  have  been  taken  into  account.  If  the  strat- 
ified rocks  at  the  bottom  of  a  series  of  strata,  apparently  accumu- 
lated to  vast  thicknesses,  are  of  a  character  similar  to  those  much 
higher  up,  or  near  the  top  of  the  series,  it  is  improbable,  for  several 
reasons,  that  the  whole  was  ever  more  than  15,000-20,000  feet  thick. 

An  error  may  be  made  in  estimating  the  maximum  thickness  of  strata,  in 
that  the  initial  inclination  to  the  horizontal  and  the  overlap  of  the  beds,  men- 
tioned above,  have  not  been  sufficiently  regarded.  This  has  been  recently 
urged  by  Chamberlin  and  Salisbury,  and  the  effect  of  it  in  causing  error  is 
illustrated  in  Fig.  217.  On  the  other  hand,  this  idea  must  be  used  with  cau- 
tion, for  it  assumes  that  in  a  thick  formation  the  beds  are  chiefly  foreset 
ones,  deposited  with  a  considerable  angle  of  inclination,  but,  in  a  broad  and 
thick  formation  made  by  a  subsidence,  the  topset  beds  are  the  dominant  ones, 
and  these  are  deposited,  as  a  rule,  with  exceedingly  gentle  angles  of  inclination. 
If  the  tilting  of  the  upturned  and  eroded  beds  is  steep,  there  is  less  error  intro- 
duced by  the  customary  method  of  estimating  their  thickness,  while  if  they 
are  only  gently  inclined,  the  idea  involved  in  the  figure  may  be  taken  into 
account. 


296  TEXT-BOOK   OF   GEOLOGY 

Relative  Age  of  Beds.  —  In  a  series  of  beds  of  horizontal  strata 
piled  upon  one  another,  we  assume  that  the  higher  a  bed  is  in  the  set, 
the  younger  it  is  in  point  of  age.  This  seems  so  obvious  as  to  need 
no  further  demonstration,  yet  it  is  the  basis  of  all  our  deciphering  of 
geological  history,  and  upon  it,  in  fact,  the  science  of  evolutionary 
paleontology  has  been  built.  This  substructure  of  geology  is  so 
humble  and  close  to  the  ground,  that  it  is  often  overlooked  in  the 
contemplation  of  the  edifice  which  has  been  erected  upon  it;  for  the 
beginner,  especially,  it  should  not  be  lost  sight  of.  If  we  should  con- 
clude, however,  that  the  rule  is  invariable,  in  all  places  and  under 
all  conditions,  that  an  upper  stratum  is  younger  than  one  appearing 
below  it  in  horizontal  position,  we  should  fall  into  serious  error,  for 
there  are  some  remarkable  exceptions  to  it,  as  will  be  shown  later  on. 

Deformation  of  Strata 

Although  over  wide  regions  the  stratified  rocks  appear  to  be  lying, 
in  a  general  way,  in  the  horizontal  position  in  which  the  beds  were 
laid  down,  and  to  have  been  raised  to  form  land  surfaces,  sometimes 
to  great  heights,  without  serious  displacements,  it  is  a  matter  of 


Fig.  218.  —  Section  12  miles  long  showing  folding  in  the  Appalachian  Mountains, 
near  Greeneville,  Tenn.     Modified  from  Keith  and  Willis. 

common  observation  that  in  other  places,  notably  in  mountain 
regions,  this  is  not  the  case.  Here,  on  the  other  hand,  we  find  the 
strata  inclined,  often  at  high  angles,  thrown  into  folds,  contorted, 
bent  and  often  more  or  less  broken.  All  such  displacements  may  be 
included  under  the  term  of  deformation  of  strata,  which  will  now  be 
described.  The  discussion  of  the  most  probable  causes  for  this  phe- 
nomenon will  be  deferred  until  later,  when  mountain  ranges,  in 
which  it  is  so  prominently  displayed,  are  treated;  only  the  struc- 
tures themselves  being  here  considered. 

Folds :  Anticline  and  Syncline.  —  Study  of  the  strata,  by  methods 
to  be  presently  mentioned,  shows  that  in  many  places  they  have 
been  corrugated  into  folds.  Sometimes  these  are  on  a  small  scale 
and  can  be  readily  seen,  Figs.  221  and  222;  often  the  folding  is  on 
such  a  great  scale  that  only  here  and  there  in  cliffs,  rock  outcrops, 
etc.,  are  portions  of  a  fold  exposed,  which  in  the  strata  simply  appear 
inclined,  but  by  noting  the  inclination  and  following  the  outcrops  of 
particularly  distinguishable  layers,  sometimes  for  miles,  the  im- 


SEDIMENTARY   ROCKS  297 

mense  size  of  the  folds  becomes  clear  to  us.  The  manner  in  which 
strata  have  been  folded,  and  the  scale  on  which  it  occurs,  may  be 
seen  by  inspection  of  the  adjoined  section,  Fig.  218,  which  has  been 
worked  out  by  geologists. 

With  respect  to  folding,  two  terms  are  constantly  used  by  geolo- 
gists. In  a  series  of  folds,  it  is  evident  that,  like  waves,  they  consist 
of  alternate  crests  and  troughs.  The  crests  of  the  folds  are  termed 
anticlines,  while  the  troughs  are 
called  synclines.  This  is  shown  in 
Fig.  219,  where  the  up-folds  A  are 
the  anticlines,  and  the  down-folds 
S,  the  synclines.  Even  if  through  Fig' 21,9'  -Dia^am  to  illustrate  anti- 

.    .  clmes,  A,  and  synclines,  S. 

erosion  the  original   crests   should 

be  carried  away,  and  the  whole  reduced  to  a  level  surface  II,  we 
should  still  term  the  A  portions  below  the  surface  anticlines  and  the 
S  portions  synclines,  and  in  imagination  reconstruct  the  missing 
parts.  This  should  make  it  clear  that  anticlines  and  synclines  are 
not  a  matter  of  surface  topography,  but  of  structure.  Although,  as 
not  infrequently  happens,  the  original  configuration  of  the  surface 
may  be  reversed  by  erosion,  as  in  Fig.  220,  we  should  call  the  parts 
S,  of  the  down-folds  still  left,  synclines,  and  the  up-folds  between, 
anticlines,  and  infer  the  underground  structure  shown.  A  natural 
section  through  an  anticline  is  seen  in  Fig.  221,  and  one  through  a 
syncline  in  Fig.  222. 


Fig.  220.  —  Anticlines,  A,  and  synclines,  S. 

A  simple  mnemonic  method,  in  regard  to  anticline  and  syncline,  to  enable 
the  beginner  to  remember  which  is  which,  is  to  notice  that  the  form  of  the 
letter  A,  the  initial  one  of  anticline,  in  itself  represents  a  sharp-crested  anti- 
cline. 

Outcrop.  —  In  considering  the  deformation  of  strata,  only  the 
simplest  case  has  so  far  been  presented,  where  they  have  been 
thrown  into  a  series  of  simple,  upright,  regular  folds.  But,  while 
this  sometimes  happens,  it  is  usual  to  find  the  nature  of  the  folding 
much  more  complicated  than  this,  and  there  are  also  other  things 
in  regard  to  folds  which  are  of  importance,  besides  the  consideration 

•of  a  simple  section  across  them.  The  varied  kinds  of  deformation 
(or  lack  of  it) ,  which  the  strata  have  suffered  in  any  region,  condi- 
tion the  geologic  structure  of  that  region,  and  it  is  a  matter  of  the 


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Fig.  221.  —  An  anticline,  broken  at  the  top.  In  the  foreground  the  outcrops  of  the 
eroded  strata  are  seen  dipping  outwardly,  from  which  the  anticline  structure  could 
be  inferred  if  the  arch  did  not  exist.  Pembroke,  Wales.  Geol.  Surv.  of  England 
and  Wales. 

highest  importance,  in  several  ways,  as  we  shall  presently  see,  that 
the  geologic  structure  should  be,  so  far  as  possible,  known  for  every 
country.  If  the  surface  of  the  earth  were  everywhere  naked  bed- 
rock, this  would  not  be,  relatively,  so  difficult ;  but  since  it  has  been 


Fig.  222.  —  A  syncline,  near  Hancock,  Md.     C.  D.  Walcott,  U.  S.  Geol.  Surv. 


SEDIMENTARY    ROCKS 


299 


greatly  eroded,  and  is  so  covered  with  earth  and  vegetation,  or 
water,  snow  and  ice,  the  difficulties  of  the  task  have  been  enor- 
mously augmented.  The  manner  in  which  we  are  able  to  discover  the 
structure  in  a  region  is  by  a  careful  study  and  comparison  of  the 
outcrops,  and  by  this  term  is  meant  those  places  where  the  under- 
lying bed-rock  comes  to  the  surface  and  is  exposed.  See  Fig.  221. 
If  the  ground  were  perfectly  level 
and  the  strata  horizontal,  the  out- 
crop would  be  the  flat  surface  of  a 
rock  stratum,  and  we  should  learn 
little  from  it,  beyond  that  fact. 
But  if  the  ground  is  cut  in  any 
way,  as  by  streams,  we  might  be 
able  to  inspect  the  outcropping 
edges  of  the  strata  along  the  valley  slope,  as  in  Fig.  205.  If  the 
sides  of  the  valley  were  trenched  by  ravines,  the  line  of  outcrop 
would  not  be  straight,  but  sinuous,  retreating  from  the  valley  into 
the  ravines,  and  advancing  on  the  spurs.  See  Fig.  223. 

If  the  strata  have  been  inclined  by  folding,  and  eroded,  it  will  fre- 
quently happen  that  the  edges  of  the  harder,  more  resistant  beds 
outcrop  in  projecting  rock  masses,  or  reefs.  In  mountain  regions, 


Fig.  224.  —  To  illustrate  dip  and  strike     Fig.  225.  —  Illustrating  outcrop,  strike,  dip, 
of  strata.  and  determination  of  latter  by  clinometer. 

the  higher  up  we  go,  the  less  soil  there  is  apt  to  be,  and  the  more 
outcrops,  until  eventually  the  rocky  ridges  themselves  form  vast 
outcrops.  By  study  of  the  dip  and  strike  of  the  outcrops  of  a 
region  the  geologic  structure  is  determined. 

Dip  and  Strike.  —  These  terms  may  be  defined  as  follows:  Dip 
is  the  angle  of  inclination  of  the  plane  of  stratification  with  the  hori- 
zontal plane.  Strike  is  the  direction  of  the  line  of  intersection  of  the 
plane  of  stratification  with  the  horizontal  plane.  This  may  be  illus- 
trated by  Fig.  224.  Imagine  XYZ  to  be  the  horizontal  plane,  and 
ABCD  the  plane  of  stratification  of  the  inclined  strata.  Then  the 
angle  DBF  =  HBG  is  the  dip,  and  the  direction  of  the  line  AB,  re- 


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Fig.  226.  —  Use  of  dip  and  strike. 


f erred  to  north  and  south,  is  the  strike.  Since  two  planes  could  be 
passed  through  AB,  one  dipping  to  the  left,  as  in  Fig.  224,  the 
other  dipping  to  the  right,  with  equal  angles  of  inclination,  it  is 
necessary  to  give  the  direction  of  dip  to  know  which  one  is  meant. 
Thus  in  a  case  like  that  shown  in  the  diagram  one  would  say,  strike 
N.  30°  W.,  dip  25°  S.  60°  W.  Since  the  lines  of  direction  of  dip  and 
strike  are  always  at  right  angles,  it  is  not  really  necessary  to  give 
the  strike,  if  the  direction  and  angle  of  dip  are  known;  thus,  dip 
25°  S.  60°  W.,  would  be  enough. 

The  direction  of  strike  is  taken  with 
a  compass  furnished  with  sights,  or 
whose  containing  box  or  bed-plate  has 
straight  edges  aligned  parallel  to  the 
NS  points.  The  dip  is  taken  with  an 
instrument  termed  the  clinometer,  a 
pendulum  swinging  over  a  graduated 
arc,  which  measures  the  angle  of  in- 
clination. The  mode  of  use  is  shown  in 
Fig.  225.  For  geologic  purposes  the 
compass  and  clinometer  are  usually  combined  in  one  instrument.  The  deter- 
mination of  dip  and  strike  is  not  only  necessary  to  unravel  the  geologic  struc- 
ture of  a  region,  but  has  a  practical  use  in  other  directions.  Suppose  that 
ABCD  in  Fig.  226  represents  the  boundaries  of  an  area  of  land,  which  is  known 
to  contain  a  bed  of  coal,  marble  or  valuable  ore,  represented  by  EFG  for  ex- 
ample, and  for  commercial  or  legal  purposes  it  is  necessary  to  give  a  descrip- 
tion of  the  bed,  and  its  exact  position  with  reference  to  the  property.  The 
determination  of  its  dip  and  the  situation  of  its  strike,  with  regard  to  the 
boundaries  of  the  plot,  furnish  this  position.  The  determination  of  dip  and 
strike,  when  a  reasonably  good  face  of  the  strata  is  exposed,  as  in  Fig.  221,  is 
a  comparatively  easy  matter,  but  where  they  are  cut  obliquely  by  erosion,  and 
are  on  sloping  hillsides,  it  is  much  more  difficult;  the  plane  of  stratification 
must  be  conceived  from  the  data  furnished  by  the  outcrop,  and  the  desired 
results  obtained  by  measurement  of  the  imagined  plane.  If  the  strata  are 
horizontal,  they  have,  of  course,  neither  dip  nor  strike;  if  they  are  vertical, 
the  dip  is  90°,  the  strike,  the  compass  direction  of  the  outcrop  upon  the  hori- 
zontal plane. 

Dip  and  strike  are  represented  upon  geologic  maps  by  a  conventional  sign  T; 
in  which  the  direction  of  the  cross  bar,  as  placed  on  the  map,  indicates  the  di- 
rection of  strike,  while  the  upright  leg  points  in  the  direction  of  dip.  See  Fig. 
229.  The  length  of  the  latter  is  also  sometimes  used  to  show  the  amount  of 
dip ;  thus  H-  ,  with  long  leg,  means  a  low  angle  of  inclination,  while  h  ,  with 
short  leg,  a  very  steep  dip ;  or  the  actual  amount  in  degrees  may  be  written  in, 
thus  t_  30°. 

Discussion  of  Folds.  —  Now  that  anticlines  and  synclines  have 
been  described,  and  the  means  by  which  these  structures,  which  are 
generally  more  or  less  eroded,  are  determined  in  the  field  by  study 


SEDIMENTARY   ROCKS 


301 


of  the  dip  and  strike  of  the  available  outcrops,  it  is  in  order  to  fur- 
ther consider  the  nature  and  extent  of  folding.  It  is  quite  evident 
that  a  fold,  up-arched,  could  not  run  in  the  direction  of  strike  in- 


Fig.  227.  —  A,  Ending  of  a  syncline.     B,  Near  the  ending  of  an  anticline. 
By  H.  H.  Robinson. 

definitely,  or  around  the  world;  it  must  end  somewhere.  At  the 
ending  of  a  syncline  we  should  have  the  structure  seen  in  A,  Fig. 
227,  and  in  B,  that  of  an  anticline  approaching  its  ending,  only  a 


Fig.  228.  —  On  a  plain  of  marine  erosion  the  outcropping  edges  of  the  strata  are  seen 
at  the  ending  of  a  syncline,  as  shown  by  the  curving  strike  and  inward  dip.  Near 
North  Berwick,  Scotland.  Geol.  Surv.  of  Scotland. 

single  stratum  being  shown  in  both.  In  the  former  the  plane  of 
stratification  is  warped  into  a  form  like  the  end  of  a  canoe;  in  the 
latter  the  canoe  would  be  overturned.  It  is  evident  that  in  both,  the 


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strike  of  a  hard  projecting  bed,  as  determined  from  its  outcrops 
after  the  erosion  indicated,  would  be  elliptical,  Fig.  228.  The  line 
of  direction  xy  of  the  fold  in  Fig.  229  is  the  axis  of  the  fold;  in  the 
syncline,  at  its  end,  this  line  emerges  from  the  ground,  in  the  anti- 
cline it  plunges  into  it;  the  amount  of  inclination  to  the  horizontal 


Fig.  229.  —  Map  of  dip  and  strike  showing  underground  structure  of  A,  a 
syncline  and  B,  an  anticline. 

of  this  line  as  the  fold  dies  out  is  called  the  pitch  of  the  fold.  See 
also  Fig.  227.  Considered  in  regard  to  their  relative  length  and 
breadth,  folds  may  vary  from  a  dome-shaped  uplift  of  the  strata, 
whose  strike  would  be  circular,  to  extremely  long  narrow  anticlines, 
whose  strike  is  an  elongated  ellipse-like  outline,  and  along  whose 
sides  the  strike  of  the  outcrops  may  be  parallel  for  many  miles,  as  in 
the  Appalachian  Mountains.  And,  of  course,  the  same  may  be  true, 
in  reversed  structure,  for  synclines. 

Inclined,  Asymmetric  and   Broken  Folds.  —  In  the  cases  men- 
tioned above  we  have  considered  simple,  regular,  upright  folds.     If 


Fig.  230.  —  Upright  symmetrical  fold; 
axial  plane  vertical. 


Fig.  231.  —  Inclined  symmetrical  fold: 
axial  plane  inclined. 


through  the  center  of  a  fold  and  its  axis  a  plane  be  imagined  to  have 
been  passed,  as  in  Fig.  230,  like  the  extended  keel  of  a  boat,  we  may 
term  this  the  axial  plane  of  the  fold.  In  a  regular,  or  symmetric 
fold,  this  plane  is  one  of  symmetry,  that  is,  the  parts  to  left  and  right 
of  it  are  symmetrically  disposed,  or  each  point  on  the  left  of  the 
plane  has  its  corresponding  point  at  an  equal  distance  on  the  right 
of  it.  If  the  fold  is  upright  the  plane  is  vertical,  Fig.  230.  But  it 
is  very  common  to  find  that  folds  instead  of  being  upright  have 


SEDIMENTARY   ROCKS 


303 


been  pushed  over  as  in  Fig.  231  until  the  axial  plane  is  inclined ;  in 
this  case  the  fold  is  said  to  be  overturned.  Such  overturning  may, 
indeed,  go  so  far  that  the  axial  plane  is  nearly,  or  actually,  hori- 
zontal ;  and  the  fold  is  then  termed  recumbent.  An  example  of  an 


Fig.  232.  —  An  overturned  and  nearly  recumbent  anticline.     Panther  Gap,  Va.     N.  H. 
Darton.     U.  S.  Geol.  Surv. 

overturned  anticline  is  seen  in  Fig.  232.    Similar  cases  may  happen 
with  synclines. 

It  is  also  very  common  to  find  that  folds  are  asymmetric  (without 
symmetry) ,  that  is,  they  are  not  similar  to  right  and  left  of  the  axial 


Fig.  233.  —  An  asymmetric  fold. 


Fig.  234.  —A,  closed  fold;   B,  open  fold. 


plane,  which  is  not,  therefore,  one  of  symmetry,  as  in  a  regular  fold. 
See  Fig.  233.  Such  asymmetric  folds  may  be  upright,  overturned, 
or  recumbent,  as  with  regular  ones. 

Finally,  folds  may  be  so  sharply  flexed  (creased)  that  they  may 
break  to  a  greater  or  lesser  degree,  especially  at  the  apex.  And  on 
breaking,  the  parts  are  liable  to  be  displaced  with  respect  to  one 
another,  or  faulted.  Faulting,  however,  is  so  important  a  phenom- 


304  TEXT-BOOK  OF  GEOLOGY 

enon  that  it  deserves  especial  consideration,  which  will  be  given  to 
it  in  a  later  place. 

Other  Features  of  Folding.  —  In  addition  to  the  important  general  charac- 
ters of  folds  described  above  there  are  some  others,  which,  at  times  and  in 
places,  are  of  such  interest  that  they  deserve  mention.  Thus,  when  folds  are 
so  sharply  flexed  that  the  side  limbs  are  in  contact,  as  in  A,  Fig.  234,  they 
are  said  to  be  closed;  in  this  case  the  horizontal  distance  across  the  strata, 
or  the  width  of  the  fold,  cannot  be  further  reduced  without  squeezing  or 
mashing  of  the  beds;  when  not  in  contact,  as  in  B,  they  are  said  to  be  open 


Fig.  235.  —  Outcrop  of  strata  show  as  in  a;  they  might  be  one  series  with  inclined 
dip,  or  they  may  be  closed  isoclinal  folds  and  receive  various  explanations,  two 
possible  ones  being  shown  in  6  and  c. 

and  the  strata  may  be  further  folded  without  mashing.  An  example  of  a 
closed  fold  is  seen  in  the  overturned  anticline,  Fig.  232;  a  diagram  of  open, 
upright  anticlines  in  Fig.  220. 

In  isoclinal  (equal  inclination)  folds  the  strata  are  compressed  until,  on  both 
sides  of  a  fold,  and  perhaps  throughout  a  series,  they  are  parallel  and  have  the 
same  dip,  Fig.  235,  b  and  c.  When  such  folds  are  cut  away  by  erosion  as  in  a, 
they  may  form  structures  very  difficult  of  interpretation. 

For  a  series  of  strata  with  a  more  or  less  uniform  dip,  the  original  struc- 
ture of  which  for  any  reason  is  in  doubt,  Daly  has  proposed  the  term 
homocline,  which  is  defined  as  any  block  or  mass  of  bedded  rocks  all  dipping 
in  the  same  direction. 

In  what  has  been  said  so  far  of  folds  they  have  been  treated  as  simple 
structures  with  true  axial  planes.  But  folds  are  frequently  warped  or  bent, 
so  that  the  flat  axial  plane  of  the  regular  fold  is  also  warped  or  bent,  not 
only  in  vertical,  but  also  in  about  horizontal  directions.  Folds  may  also  branch 
or  be  compound,  and  thus  a  variety  of  most  complicated  structures  be  in- 
duced. And  this  is  true  both  of  anticlines  and  synclines.  See  Fig.  236. 

Sometimes  in  the  elevation  of  portions  of  a  country,  as  in  the  Plateau  region 
of  the  Southwest,  the  strata  are  flexed  as  shown  in  Fig.  237.  This  is  known 
as  a  monocline.  Strictly  defined,  it  is  a  one  limb  flexure,  on  either  side  of  which 
the  strata  are  horizontal  or  have  uniform  gentle  slopes.  True  monoclines  are 
uncommon,  and  many  that  have  been  described  as  such  are,  in  reality,  very 
asymmetrical  anticlines. 

Geosynclines  and  Geanticlines.  —  It  has  been  previously  men- 
tioned, page  251,  that  long  narrow  belts  of  the  ocean  floor  near  the 
continental  masses  form  concave  tracts  from  100  to  200  miles  broad. 


SEDIMENTARY   ROCKS 


305 


These  down-warps  of  the  earth's  crust  are  evidently  on  a  vastly 
greater  scale  than  the  folds  we  have  been  considering  and  which 
appear  as  mere  minor  and  superficial  wrinkles  in  comparison.  Such 


Fig.  236.  —  Map  of  an  area  near  Rome,  Georgia,  showing  warping,  branching,  and 
ending  of  folds,  both  anticlines  and  synclines,  by  the  ridges  and  lines  of  outcrop 
of  strata  on  the  surface.  Rome  Folio,  U.  S.  G.  S. ;  C.  W.  Hayes. 

great  down-warps,  which  may  be  1000  miles  long  and  200  broad, 
have  been  termed  geosynclines  by  Professor  J.  D.  Dana.  There  are 
facts  which  go  to  prove  that,  correspondingly,  there  are  broad  up- 
lifts which  are  called  geanticlines.  The  prefix  is  from  the  Greek 
word  signifying  the  earth,  to  emphasize  the  scale  of  the  phenomena. 
Such  up-  and  down-warps  may  occur  on  the  continents,  as  well  as 
on  the  ocean  floor.  Thus  the  basin  of  Lake  Superior  has  been  held 
to  represent  a  geosyncline,  while  the  country  northeast  of  it,  in 


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TEXT-BOOK   OF   GEOLOGY 


Canada,  extending  to  Labrador,  is  believed  to  be  a  low,  wide,  slowly 
rising  arch,  which  may  represent  a  geanticline  (see  page  240) .  In 
the  region  about  Cincinnati  there  is  also  a  wide  flat  arch  which 
forms  a  geanticline. 


Fig.  237.  — A  monoclinal  fold. 

The  geosynclines  of  the  past,  as  well  as  those  of  the  present,  have 
been  the  great  basins  for  the  accumulation  of  sediments,  like  those 
which  now  form  the  Appalachians  and  the  Alps.  -When  later,  by 
actions  which  we  shall  study  more  in  detail  under  mountain  ranges, 
the  accumulated  beds  are  uplifted  and  compressed  into  folds',  tHe 
whole  series  of  folds,  referred  to  the  plane  of  the  horizon,  may  form 
a  compound,  uplifted  mass,  which  erosion  thereupon  carves  into  a 


Fig.  238.  —  To  illustrate  terms  used  in  compound  folding.  The  general  uplifted 
masses  of  folds  AA  are  called  anticlinoria,  while  the  down-warped  mass  of  folds 
S  is  termed  a  synclinorium.  The  general  average  warping  effect  of  the  folding 
is  indicated  by  the  line  BE. 

mountain  range  with  the  aspect  familiar  to  us.  Such  a  mass  of 
strata,  laid  down  in  a  geosyncline  and  crushed  by  folding  into  a 
mountain  range,  has  been  termed  by  Dana  a  synclinorium  (from 
syncline  and  oros,  Greek  for  mountain). 

The  term  thus  introduced  by  Dana  has,  unfortunately,  been  diverted  from 
its  original  meaning,  and  applied  to  a  general  syncline  compounded  of  minor 
folds  and  contrasted  with  anticlinorium ;  see  Fig.  238.  -It  has  thus  become  a 
term  of  structure,  and  the  related  idea  of  mountain-making,  which  the  name 
expresses,  has  been  relegated  to  a  subordinate  position,  or  entirely  left  out. 

Unconformity  and  its  Meaning 

Definition.  —  It  is  not  uncommon  to  find,  on  examining  the 
stratified  rocks  exposed  in  outcrops  in  cliffs,  valleys,  and  mountain 
sides,  that  one  set  of  beds,  whose  parallel  position,  kinds  of  rocks 
and  their  relations,  and  contained  fossils  prove  them  a  con- 
tinuously deposited  series,  are  resting  upon  another  set  of  rocks, 
whose  position  and  characters  show  equally  well  that  they  were 


SEDIMENTARY   ROCKS 


307 


formed  at  another  period  and  under  other  conditions.  Thus  in  the 
diagram,  Fig.  239,  the  layers  of  strata  d  have  been  deposited  at 
one  period  and  under  one  set  of  conditions;  they  are,  therefore, 


-Mr 


Fig.  239.  —  Section  to  show  unconformity.      A  conformable  set  of  strata  d  rest  un- 
conformably  upon  another  conformable  set  c;  the  surface  ab  is  the  unconformity. 

spoken  of  as  a  conformable  series  of  beds,  or  a  formation.  Also 
the  series  of  beds  c  are  conformable  among  themselves;  but 
it  is  quite  evident  that  they  are  not  conformable  with  d.  To 
be  in  the  position  represented,  events  have  happened  to  them 
which  have  not  happened  to  d.  The  two  series  are  un- 
conformable  with  respect  to  one  another,  and  the  surface 


Fig.  240.  —  Angular  unconformity.  Basal  Wasatch  conglomerate  resting  on  up- 
turned and  eroded  Laramie  sandstone.  Near  Meeteetse,  Wyo.  C.  A.  Fisher, 
U.  S.  Geol.  Surv. 

ab  separating  them  is  called  an  unconformity,  see  Figs.  239 
and  240.  It  is  not  intended  by  this  statement  that  one  should 
infer  that  the  lower  rocks,  upon  which  the  upper  series  of 
strata  unconformably  rests,  should  necessarily  also  be  stratified, 
or  sedimentary  ones.  The  lower  formation  might  be  composed  of 
igneous  rocks,  such  as  granite  for  example,  or  it  might  be  composed 
of  metamorphic  ones,  such  as  schists  of  various  kinds,  but  the  surface 


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ab  would  still  exist  and  would  be  an  unconformity.  This  will  be 
better  understood  when  we  consider  the  geological  history  which  an 
unconformity  reveals. 

Geological  History  Revealed.  —  Let  us  take  a  case  like  that 
shown  in  Fig.  241,  as  an  example.  In  this  the  following  geological 
events  are  recorded.  First,  a  period  of  quiet  deposition  in  which 
the  beds  of  the  set  c  were  laid  down  in  horizontal  position,  or  nearly 


Fig.  241.  —  Angular  unconformity;  both  series  of  beds  tilted. 

so.  The  thickness  of  the  beds,  the  kinds  of  rocks,  their  character- 
istic features,  and  the  contained  fossils  constitute  the  record  for  this 
period.  Next,  a  time  of  elevation  when  the  sea-bottom  became  a 
land  surface,  accompanied  by  tilting  of  the  strata,  and  followed  by 
an  interval  when  the  uplifted  strata  were  deeply  eroded.  For  this 
period  of  uprise,  tilting,  and  erosion  we  have  no  means  of  estimat- 
ing the  duration,  except  by  the  amount  of  erosion  and  by  the  record 
of  succeeding  events  which  mark  its  end.  Not  only  were  there  no 
records  formed,  in  the  shape  of  sediments,  etc.,  but  those  of  the 
previous  period  were  wasted  and  more  or  less  lost  by  erosion.  There 
is,  therefore,  a  gap  in  the  geological  record  at  this  point,  and  it  is 
consequently  often  spoken  of  by  geologists  as  a  "lost  interval" 
Next  in  the  geological  history  followed  a  period  of  subsidence,  when 
the  eroded  surface  became  sea-bottom  again,  and  received  a  new 
deposit  of  sediments,  forming  the  conformable  series  of  strata  d  in 
our  diagram.  The  events  of  this  time  are  again  recorded  in  the 
strata  as  before.  And  finally,  after  a  second  period  of  uplift  and 
tilting,  the  whole  course  of  events  is  presented  to  us  to  be  read 
as  erosion  progresses.  The  history  here  given  may  then  be  sum- 
marized as  follows:  first,  deposition  of  strata;  second,  elevation  and 
erosion;  third,  subsidence  and  fresh  deposition;  fourth,  final  eleva- 
tion. And  as  a  corollary  we  may  add,  and  this  is  the  important 
point  about  the  matter,  that,  an  unconformity  always  represents  a 
former  more  or  less  eroded  land  surface. 

Relation  of  an  Unconformity  to  Kinds  of  Rocks.  —  Since  an  uncon- 
formity represents  a  submerged  land  surface,  it  is  of  interest  to  consider  the 
kinds  of  rocks  that  would  naturally  be  associated  with  it.  Evidence  appears 
to  prove  that  subsidence  takes  place  slowly  and,  usually,  with  more  or  less 


SEDIMENTARY   ROCKS  309 

varying  pauses  in  the  process.  As  the  land  lowers,  the  sea  works  its  way  in- 
ward, due  both  to  submergence  and  to  its  own  ceaseless  gnawing  at  the 
coast-line.  Where  the  land  and  sea  meet  there  is  generally  a  beach,  and,  as 
the  land  subsides,  this  beach  marches  inland  at  the  edge  of  the  encroaching 
sea.  Every  part  of  the  newly  made  sea-bottom  will  have  been  passed  over 
by  this  advancing  beach.  But  the  beach  is  that  part  of  the  bottom  which 
is  subjected  to  the  ebb  and  flow  of  the  tides,  and  to  the  onward  rush  of  the 
waves  and  their  returning  undertow.  Consequently,  it  is  exposed  to  the 
action  of  strong  currents,  and  only  the  coarsest  of  sediments,  gravels  and 
coarse  sands,  can  compose  it.  As  the  land  subsides,  all  its  superficial  deposits, 
earth,  stones,  and  rock  decayed  by  weathering,  will  be  worked  over  by  the 
advancing  sea,  down  to  bed-rock,  and  perhaps  deeper,  and  converted  into 
beach  materiaL  The  finer  particles  of  the  ground-up  detritus  will  be  swept 


Land  Advancing  Sea 

Earth  and  stones 

Deposits  of  quiet  water Water-level 

^^   C,   Shale 

6,  Sandstone 
tt,  Conglomerate 


Fig.  242.  —  Diagram  illustrating  the  normal  order  of  deposits  above  an  unconformity. 

out  to  sea  to  be  deposited  in  quieter  waters,  and  will  form  fine  silts  and  muds. 
If  one  now  imagines  this  state  of  affairs  gradually  passing  inland,  it  is  evident 
that  the  first  rock  stratum  of  the  new  series,  lying  unconjormably  on  the  old 
bed-rock  of  the  previous  formation,  will  be  a  conglomerate  or  coarse  sandstone, 
since  this  is  the  consolidated  form  of  the  gravels  and  sand  of  the  beach.  Above 
this  we  should  expect  the  fine  deposits  of  quiet  water  to  appear  as  finer  sand- 
stones and  shales,  representing  the  silts  and  muds,  succeeded  or  interspersed 
with  marine  deposits  of  limestone.  These  relations  are  shown  in  the  diagram, 
Fig.  242.  Normally,  then,  we  should  expect  an  unconformity  to  be  marked  by 
the  presence  of  a  conglomerate  (or  coarse  sandstone),  and  this  is  often  the  case. 
While  the  above  is  the  theory,  it  is  evident  that  in  practice,  the  result  must 
depend  largely  on  the  nature  of  the  disintegrated  land  material  supplied  to 
the  waves  and  currents,  and  the  extent  to  which  they  can  operate  on  it.  The 
theory  presupposes  that  the  material  is  of  unlike  sizes  and  hardness,  to  obtain 
the  variation  in  the  beds.  But,  if  the  land  should  consist  of  strata  of  soft 
and  homogeneous  character,  for  example,  as  seen  in  areas  composed  of  clays 
or  shales,  fine  even-grained  sediments  might  be  formed  which  would  yield  no 
basal  conglomerate.  In  the  level  interior  regions  of  North  America,  the  flat- 
lying  beds  contain  many  unconformities  which  are  not  separated  by  con- 
glomerates. 

It  is  evident  that,  if  the  process  discussed  above  were  to  be  reversed,  and  the 
sea-bottom  to  rise,  instead  of  to  sink,  the  formations  would  also  be  re- 
versed, and  the  sea-beach  would  be  the  last  deposit  left  on  the  surface  of 
the  newly  made  land.  If  we  should  imagine  this  compacted  into  rock,  it  would 
form  a  conglomerate  of  emergence,  as  contrasted  with  the  one  of  submergence, 
described  above.  Being  the  first  part  of  the  new  land  to  be  attacked  by 
erosion,  and  being  still  soft  and  unconsolidated,  it  would  be  the  first  material 
to  be  carried  away  and  to  disappear.  Hence  conglomerates  of  emergence  are 


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not  apt  to  be  found.  But  in  small  oscillations  of  the  surface  of  land  at, 
or  near,  sea-level,  a  conglomerate  of  emergence  might  persist  long  enough  to 
be  subsequently  covered  by  the  one  of  submergence.  The  two  might  then 
form,  apparently,  a  single  bed  of  conglomerate,  through  which  would  run  the 
line  of  unconformity,  which  might  thus  be  non-apparent.  This  would  furnish 
a  case  of  what  we  might  term  a  slight  unconformity.  Owing  to  the  difficulty 
of  detecting  them,  such  conglomerates  of  emergence  are  apt  to  be  overlooked 
where  they  might  occur.  This  point  will  be  referred  to  later. 


A.    Nonconformity;  lower 
formation  igneous  and 
metamorphic  rocks. 


D .     Disconf  ormity ;  evident, 
strata  horizontal. 


B.  Nonconformity;  lower 
f ormaf  ion  stratified  rocks, 
tilted. 


E.    Disconf  ormity,  evident; 
both  formations  tilted. 


C.    Nonconformity;  both  for- 
mations stratified  and  tilted. 


1    '    1    '    1    '    1    '    1 

i     i     i 

—  

"^^>^^°°^Z^?*t 

1  i  '  i  '  i  '  i  ' 

i  '  i  '  i 

i     i     i     i     i      ii 

F.    Disconf  ormity ;  nonevident, 
at  ab.  Strata  horizontal  but 
might  be  tilted. 


Fig.  243.  —  Diagram  illustrating  various  phases  of  unconformities. 

Classification  of  Unconformities.  —  These  may  be  divided  into 
two  main  groups.  In  the  first,  the  lower  formation,  either  by  the 
tilting  of  the  beds  or  by  its  being  composed  of  non-stratified  rocks, 
shows  at  once  its  non- conformity  with  the  series  of  beds  above  it. 
This  kind  is  illustrated  by  the  diagrams  (Fig.  239  and  241)  pre- 
viously given.  It  is  sometimes  called  an  angular  unconformity 
(see  Fig.  240) ,  because  the  stratification  planes  of  the  two  series  are 


SEDIMENTARY   ROCKS  311 

at  an  angle ;  this  term  is  good  so  far  as  it  goes,  but  it  does  not  cover 
the  whole  case,  since,  as  mentioned  above,  the  lower  formation  is 
not  always  composed  of  stratified  rocks  with  distinct  stratification 
planes,  but  may  be  of  igneous  or  metamorphic  rocks,  which  do  not 
show  them.  A  more  general  term  is  needed,  and  an  unconformity 
of  this  class  is  here  termed  a  nonconformity. 

On  the  other  hand,  it  may  happen  that  the  lower  formation  will 
be  elevated,  eroded,  and  submerged  without  material  disturbance  of 
the  position  of  the  beds.  In  this  case  the  old  and  the  new  forma- 
tions will  have  their  stratification  planes  actually,  or  practically, 
parallel.  This  constitutes  an  unconformity  of  the  second  class,  and, 
as  it  is  desirable  for  a  number  of  reasons  that  it  should  be  distin- 
guished from  one  of  the  first  class,  it  has  been  termed  a  discon- 
jormity.  Further,  it  may  be  that  the  erosion  line  between  the  two 
formations  is  an  irregular  one,  and  thus  clearly  visible;  this  may  be 
termed  an  evident  disconformity.  Or,  owing  to  circumstances,  the 
erosion  line  may  run  parallel  with  the  strata  and  not  be  apparent; 
the  unconformity  must  here  be  determined  by  the  sudden  change 
in  the  nature  of  the  fossils  and  by  other  things,  such  as  the 
occurrence  of  thin  basal  sandstones  and  conglomerates,  etc.  (See 
in  this  connection  what  has  been  previously  said  regarding  con- 
glomerates of  emergence.)  In  contrast  with  the  previous  one  a 
case  of  this  kind  can  be  called  a  non-evident  disconformity.  We 
have,  then,  the  following  cases  of  unconformity: 

Unconformity. 

1.  Nonconformity,  two  formations  visibly  different. 

a.  Lower  formation  of  rocks  non-stratified,  or  apparently  so. 

b.  Lower  formation  of  stratified  rocks,  tilted. 

2.  Disconformity,  two  formations  in  parallel  position. 

a.  Evident,  erosion  line  clearly  visible. 

b.  Non-evident,  erosion  line  not  visible. 

These  different  cases  are  illustrated  in  Fig.  243. 

Also  it  may  be  added  that  in  importance,  in  the  length  of  time 
represented,  in  the  greatness  of  change  of  life  indicated  by  the  fos- 
sils, and  in  the  interest  and  variety  of  events  recorded,  the  uncon- 
formities, in  a  broad  general  way,  diminish  from  top  to  bottom  of 
the  diagram  from  A  to  F.  Disconformities  are  usually  ascertained 
on  the  basis  of  marked  changes  in  the  faunas  or  in  the  sedimenta- 
tion. There  are,  however,  many  short  breaks  in  the  sedimentary 
record,  due  largely  to  oscillations  in  the  intensity  of  climatic  fac- 
tors. In  the  aggregate  their  time  value  is  large.  These  small  breaks 
Barrell  has  called  diastems. 


CHAPTER  XII 
THE  IGNEOUS  ROCKS 

The  igneous  form  a  second  great  division  of  the  different  kinds 
of  rocks  which  make  up  the  crust  of  the  globe.  As  their  name  im- 
plies, the  presence  of  heat  is  an  essential  factor  in  their  origin,  and 
they  may  be  defined  as  those  rocks  which  have  been  formed  by  the 
solidification  of  molten  masses  from  within  the  earth.  Such  molten 
masses,  as  has  been  mentioned  under  volcanic  action,  page  200,  are 
commonly  called  magmas,  a  term  we  shall  frequently  use  in  speak- 
ing of  them.  These  rocks  are  sometimes  referred  to  as  primary,  be- 
cause the  material  which  composes  the  other  kinds  was  originally 
derived,  either  from  them  or  from  the  original  shell  of  the  earth, 
which  many  regard  to  have  been  of  the  nature  of  igneous  rock. 

Distinguishing  Characters.  —  The  features  of  igneous  rocks  by 
which  they  may  be  distinguished  from  the  sedimentary  ones,  whose 
characters  have  been  given  in  preceding  pages,  and  from  the  meta- 
morphic  ones  to  be  presently  described,  consist  partly  in  the  rela- 
tions which  the  masses  exhibit  towards  other  rocks  and  which  we 
may  term  their  mode  of  occurrence,  and  partly  in  characters  which 
become  evident  when  a  rock  mass,  or  a  piece  of  it,  is  closely  ex- 
amined. The  different  modes  of  occurrence  of  the  igneous  rocks 
will  be  described  in  the  following  section;  with  regard  to  those 
more  minute  features  which  are  seen  on  close  examination,  the  fol- 
lowing are  of  importance.  The  igneous  rocks  do  not,  of  course, 
contain  fossils,  nor,  as  a  rule,  do  they  show  the  parallel  or  banded 
appearance  of  the  stratified  rocks,  for,  in  general,  a  surface  in  one 
direction  looks  like  a  surface  in  any  other  direction.  They  also 
have  certain  peculiarities  in  the  minerals  composing  them,  and  in 
the  arrangement  of  these  mineral  grains  (the  texture,  so-called), 
which  distinguish  them.  Sometimes,  indeed,  they  are  more  or  less 
made  of  glass,  which  at  once  betrays  their  origin,  since  this  substance 
could  be  formed  only  by  the  chilling  of  melted  material.  These 
features  will  be  more  fully  described  when  the  different  kinds  of 
igneous  rocks  and  their  classification  are  considered;  we  will  first 
discuss  the  various  ways  in  which  masses  of  igneous  rock  occur. 

312 


THE   IGNEOUS   ROCKS  313 

Occurrences  of  Igneous  Rocks 

Intrusive  and  Extrusive  Rocks.  —  There  are  two  chief  modes  of 
occurrence  of  igneous  rocks,  the  intrusive  and  the  extrusive.  In 
the  former  the  magma,  rising  from  depths  below,  has  stopped 
before  attaining  the  surface  and  has  cooled  and  solidified,  sur- 
rounded by  other  rock  masses  of  the  earth's  outer  shell.  In  the 
extrusive,  the  magma  has  attained  the  surface,  come  out  upon  it, 
and  there  solidified,  forming  the  rock  masses.  These  are  some- 
times called  effusive  and  sometimes  volcanic  rocks,  though  it  is  held 
by  some  that  they  are  not  always  connected  with  volcanoes.  See 
page  221. 

It  should  be  understood  that,  although  the  division  of  igneous  rocks  into 
intrusive  and  extrusive  is  a  natural  one,  the  two  are  closely  connected 
and,  in  fact,  pass  from  one  into  the  other.  The  magma  forming  every  ex- 
trusive mass  has  come  through  some  passageway  below,  which  has  remained 
filled,  and,  eventually,  solidified  into  rock.  There  is  thus  a  prolongation  of 
every  extrusive  body  above,  into  an  intrusive  one  below.  In  some  cases 
this  connection  of  the  extrusive  with  its  root  below  may  be  seen,  but,  more 
generally,  the  extrusive  covers  it,  or  has  been  separated  from  it  by  erosion, 
and  the  rock  continuity  has  been  lost.  It  is  clear  also  that  we  must  con- 
ceive the  intrusive  prolongation  as  passing  downward  into  some  greater  mass 
of  magma  (or  rock)  below,  of  a  nature  to  be  presently  described.  See  also, 
in  this  connection,  what  has  been  said  regarding  the  relation  between  vol- 
canoes and  deep  masses  of  magma,  page  200. 

With  both  intrusive  and  extrusive  rocks  there  are  variations  in 
the  mode  of  occurrence  depending,  in  the  case  of  the  former,  on 
the  relations  an  intrusive  mass  may  bear  to  the  other  rocks  which 
enclose  it,  and  in  the  extrusive  on  the  conditions  under  which  the 
magma  was  ejected.  Following  the  course  of  the  magma  upward 
we  will  begin  with  the  intrusive ;  but  it  should  first  be  recalled  that, 
since  these  rock  masses  were  covered  by  previously  existent  ones 
at  the  time  of  their  formation,  they  can  only  be  exposed  at  the 
surface,  and  thus  laid  open  to  observation,  after  a  period  of  erosion 
sufficient  to  carry  away  the  cover  and  disclose  their  intrusive  rela- 
tions. In  some  cases,  when  intruded  near  the  surface,  this  time  in- 
terval may  have  been  a  comparatively  short  one ;  in  other  cases  very 
prolonged,  when  the  masses  were  deeply  buried. 

Intrusive  Modes  of  Occurrence.  —  These  are  dikes,  intrusive 
sheets,  laccoliths,  necks,  stocks,  and  bathyliths.  Several  other 
modes  of  occurrence  have  been  described  and  named,  but  as  they 
have  not  yet  been  generally  recognized  as  of  the  importance  of  those 
mentioned,  they  will,  for  simplicity's  sake,  be  treated  as  modifica- 


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tions  of  them.     The  simplest  form  of  intrusion  is  that  of  the  dike 
and  this  will  be  considered  first. 

Dikes.  —  A  dike  results  from  solidification  of  molten  magma  after 
it  has  entered  a  simple  fissure  in  preexistent  rocks.  Consequently, 
its  extension  in  length  and  breadth  is  great  compared  with  its  thick- 
ness. It  may  "  cut,"  that  is,  pass  through,  rocks  of  any  kind,  igneous, 
sedimentary,  or  metamorphic;  but  in  the  sedimentary  it  must  traverse 


Fig.  244.  —  Dike  of  trap-rock  in  granite.  In  this  case  the  dike  is  less  resistant 
and  has  been  cut  away  by  erosion,  leaving  a  trench  in  the  granite.  Isles  of 
Shoals,  N.  H. 

the  planes  of  stratification  at  an  angle;  if  parallel  to  them,  it  is  an 
intrusive  sheet.  As  exposed  on  the  surface  it  may  be  a  few  yards,  or 
many  miles,  long;  it  may  be  a  fraction  of  an  inch,  or  many  hundreds, 
or  even  some  thousands  of  feet  in  thickness.  An  illustration  of  a 
dike  is  seen  in  Fig.  244. 

The  ordinary  thickness  of  most  dikes  is  from  two  or  three  feet  up  to  twenty ; 
the  exposed  length  very  variable.  The  course  of  a  great  dike  in  the  north  of 
England  has  been  traced  for  over  100  miles.  As  we  naturally  think  of  a  bed  of 
stratified  rock  as  in  a  horizontal  position  and  call  its  departure  from  horizontal- 
ity  its  dip,  so  we  think  of  a  dike  as  a  sheet  of  rock  in  a  vertical  position;  this 
is  by  no  means  always  the  case,  and  the  angle  of  inclination  of  the  plane  of 
extension  of  the  dike  with  the  vertical  is  called  its  hade.  The  direction  of 
its  outcrop,  or  intersection  with  the  horizontal  plane,  is  termed  its  strike,  or 
trend. 

Dikes  may  have  attained  the  surface  and  given  rise  to  outflowings  of  lava, 
or  they  may  not  have  reached  it,  but  have  been  exposed  by  later  erosion.  In 
some  cases  they  have  formed  the  canals  feeding  larger  intrusive  bodies  above 


THE    IGNEOUS    ROCKS 


315 


them,  such  as  the  sheets  and  laccoliths  to  be  next  described.  In  the  process  of 
erosion,  it  sometimes  happens  that  the  dike  is  more  resistant  than  the  sur- 
rounding rocks  and  is  left  projecting  as  a  wall;  sometimes  less  resistant  and 
has  become  a  ditch;  from  these  features  the  name  is  derived,  especially  the 
more  prominent  wall,  for  dike  means  both  wall  and  ditch.  The  rock  of  a  dike 
is  cut  into  blocks  by  fissures,  and  very  commonly  the  blocks  are  columns 
lying  perpendicular  to  the  wall  of  the  dike,  like  a  pile  of  cordwood,  an  ar- 
rangement whose  origin  is  described  later  under  columnar  structure.  Dikes 
occur  in  many  places  in  more  or  less  well-defined  systems,  and  around  vol- 
canic centers  are  apt  to  be  radially  disposed. 


Fig.  245.  —  Intrusive  sheets  of  igneous  rock.     Cottonwood  Canyon,  New  Mexico. 
W.  T.  Lee,  U.  S.  Geol.  Surv. 

Intrusive  Sheets.  —  It  is  not  uncommon  to  find,  where  intrusions 
of  magma  occur  in  stratified  rocks,  that  it  has  been  forced  in  layers 
between  the  beds.  This  most  frequently  happens  where  the  beds 
are  weak  and  easily  penetrated,  as  in  shales,  thinly  bedded  sand- 
stones, and  the  like.  Such  a  flat  extended  mass  lying  concordantly 
along  the  planes  of  stratification  is  known  as  an  intrusive  sheet  of 
igneous  rock,  though  sometimes  it  is  spoken  of  as  a  sill.  Such  sheets 
may  be  a  foot  or  so  in  thickness,  or  several  hundred  feet,  and  they 
may  spread  over  many  miles  in  area.  An  illustration  of  them  is 
seen  in  Fig.  245. 

Sheets  may  break  dike-like  across  the  strata  and  be  continued  along  a  new 
horizon.  These  intrusions  may  be  distinguished  from  surface  flows  of  lava, 
which  have  been  buried  by  deposits  of  later  sediments,  by  the  rock  composing 
them  being  of  the  same  hard  firm  nature  at  top  and  bottom,  and  by  the  over- 
lying sediments  being  baked  and  altered  by  the  intrusion.  The  surface  of  a 


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lava  flow  is  usually  spongy,  ropy,  slaggy,  etc.  (see  page  205),  and  a  flow  could, 
of  course,  exert  no  action  on  beds  not  yet  deposited  upon  it.  Intrusive  sheets 
are  most  liable  to  occur  where  larger,  more  important  intrusions  of  magmas, 
such  as  laccoliths  and  stocks,  have  taken  place,  as  accompanying  features  in 
the  surrounding  strata.  In  regions  where  thick  intrusive  sheets  occur,  and  the 
strata  have  been  broken,  dislocated  and  upturned  by  faulting  (see  faulting), 
they  may  give  rise  to  prominent  topographic  and  scenic  land  features  through 
the  effects  of  later  erosion.  This  is  illustrated  in  some  of  the  trap  ridges  of 
southern  New  England,  northern  New  Jersey,  and  in  other  places. 


Fig.  246.  —  Section  of  a  laccolith.     The  black  area  is  the  igneous  rock. 

Laccoliths.  —  The  laccolith  in  its  typical  development  is  a  lentic- 
ular or  dome-shaped  mass  of  magma  intruded  into  the  sedimentary 
rocks  between  the  bedding  planes.  It  has  a  flat  floor,  and  is  more 


• 


Fig.  247.  —  Bear  Butte;  a  laccolith  denuded  of  its  cover  and  the  igneous  mass  laid 
bare.  The  ring  of  upturned  eroded  strata  is  seen  about  its  base.  Black  Hills, 
South  Dakota.  N.  H.  Darton,  U.  S.  Geol.  Surv. 

or  less  circular  in  ground  plan.  If  the  supply  of  material  in  the 
formation  of  an  intrusive  sheet  is  more  rapid  from  below  than  can 
easily  spread  laterally,  the  strata  above  will  be  uparched,  as  if  by 
a  hydrostatic  press,  and  a  thick  lens  of  liquid  rock  will  be  pro- 
duced, giving  rise  on  solidification  to  a  laccolith.  The  name  is 


THE    IGNEOUS   ROCKS 


317 


from  the  Greek,  meaning  cistern-rock.  Such  a  mass  may  be  a  few 
hundreds  of  feet  or  a  mile  thick  at  the  center,  and  a  few  hundreds 
of  yards  or  several  miles  in  diameter.  In  the  uprise,  the  sedi- 
mentary beds  above  are  usually  more  or  less  stretched,  thinned, 
and  broken.  A  section  of  a  laccolith  is  shown  in  Fig.  246,  and  a 
photograph  of  one  from  which  the  cover  has  been  removed  by 
erosion,  laying  bare  the  mass  of  igneous  rock,  is  seen  in  Fig.  247. 


Fig.  248.  —  Strata  being  folded  by  compression  CD,  relief  from  pressure 
from  overlying  beds  and  spreading  might  occur  in  direction  AB. 

While  the  above  statement  gives  the  idea  of  a  typical  laccolith,  many  de- 
partures from  this  arrangement  are  found  in  the  actual  occurrences.  In  ground 
plan  they  may  be  circular,  oval,  or  quite  irregular,  and  instead  of  being  symme- 
trical in  section,  as  in  Fig.  246,  they  may  be  one-sided,  or  wedge-shaped. 
According  to  their  degree  of  flatness,  all  transitions  into  intrusive  sheets  occur. 
They  may  also  break  across  the  strata  in  places  like  intrusive  sheets.  They 
may  thin  out  into  intrusive  sheets,  or  be  accompanied  by  them  on  the 
flanks  of  the  arches,  and  thus  be  compound  in  structure.  Such  sheets  may 
themselves  swell  out  into  inclined  lenticular  masses,  or  subordinate  laccoliths. 
And  in  regions  where  strata  are  being  folded,  areas  of  relief  from  pressure  or, 
possibly,  openings  might  form  on  the  sides  of  the  arches,  see  Fig.  248,  which 
would  permit  the  entrance  of  magma.  This  would  give  rise  to  inclined, 


Fig.  249.  —  Section  of  an  inclined  laccolith,  or  phacolite. 

doubly  convex  bodies  like  that  shown  in  Fig.  249.  Laccolithic  bodies  of  this 
character  have  been  termed  phacolites  by  Barker  (from  the  Greek  words  for 
lentil  +  stone).  It  is  sometimes  questioned  whether  the  magma  supplies  its 
own  force  in  making  the  intrusion  and  in  arching  up  the  strata,  or  in  other 
words  is  aggressive,  or  whether,  as  indicated  above,  the  force  lifting  the  beds 
comes  in  some  other  way,  and  the  magma  simply  flows  into  the  space  opening 
for  it,  or  the  intrusion  is,  so  to  speak,  permissive.  But  a  study  of  the  occur- 
rences shows  that  in  all  probability  both  of  these  cases  occur.  In  central 
Montana,  where  the  strata  are  horizontal  and  undisturbed  save  by  the  intru- 
sions, the  magma  must  have  acted  aggressively,  but  in  other  places  where 
folding  and  uplifts  occur,  the  intrusions  were  probably  permissive. 
Laccoliths,  more  or  less  exposed  by  erosion,  are  conspicuous  features  in 


318  TEXT-BOOK   OF  GEOLOGY 

many  parts  of  western  North  America,  where  they  were  first  discovered  by 
Gilbert  in  the  Henry  Mountains  in  southern  Utah.  Some  of  the  best  ex- 
amples occur  in  Colorado,  as  described  by  Cross,  and  they  are  also  abundant 
in  Montana.  In  places  they  are  so  aggregated  as  to  make  mountain  groups. 
More  recently  they  have  been  found  in  various  parts  of  the  world  and  thus 
appear  to  be  a  not  uncommon  form  of  intrusion. 

Bysmaliths.  —  It  may  happen  that  an  intrusion  is  so  aggressive  that  what 
would  otherwise  probably  be  a  laccolith  may  have  its  roof  ruptured,  and 
driven  upward,  by  the  magma  rising  like  a  plug  through  the  strata.  The 
vertical  dimensions  of  such  a  mass  may  be  much  greater,  compared  with  the 
lateral  ones,  than  in  a  laccolith.  A  core-like  intrusion  of  this  character  has 
been  termed  by  Iddings  a  bysmalith  (Greek,  plug-rock). 

Chonoliths,  Etc.  —  Intrusions  of  magma  may  be  of  very  irregular  shape  and 
bear  no  definite  relation  to  the  stratified  beds  in  which  they  occur,  as  do 


Fig.  250.  —  Section  through  a  partly  eroded  volcano,  with  volcanic  neck  left 

projecting. 

sheets,  dikes,  and  laccoliths.  They  may  be  formed  aggressively  by  the  rising 
magma  having  ruptured  and  crowded  aside  the  beds,  or  in  regions  of  dis- 
located rocks  by  its  having  passively  risen  into  irregular  chambers. 
For  all  such  irregularly  shaped  bodies  of  injected  rock  Daly  has  proposed 
the  name  of  chonolith  (from  the  Greek  words  meaning  "mold,"  used  in  cast- 
ing metals,  and  rock). 

Also  the  term  ethmolith  has  been  suggested  when  the  intrusive  mass  has 
a  funnel  shape,  and  lopolith  when  flattish  and  depressed,  saucer-like,  toward 
the  center.  But  none  of  these  special  designations,  except  laccolith,  has  as 
yet  acquired  general  usage. 

Necks.  —  When  a  volcano  becomes  extinct,  the  column  of  magma, 
occupying  the  conduit  leading  to  unknown  depths  below,  may 
solidify  and  form  a  mass  of  igneous  rock.  Erosion  may  cut  away 
a  great  part  of  the  ashes  and  lavas  of  the  cone,  leaving  this  more 
solid  and  resistant  rock  projecting,  as  shown  by  the  line  abc  in 
Fig.  250.  Or,  the  level  of  erosion  may  descend  into  the  rocks  which 
form  the  basement  on  which  the  volcano  is  built,  all  of  the  ashes 
and  lavas  being  swept  away,  and  only  this  mass  being  left  to  mark 
its  former  site.  Such  a  mass  of  rock  is  known  as  a  volcanic  neck, 
and  a  view  of  one  may  be  seen  in  Fig.  251.  It  is  commonly  more  or 
less  circular  in  ground  plan  and  may  be  from  a  few  hundred  yards 
up  to  a  mile  or  more  in  diameter.  The  rocks  about  volcanic  necks 
are  apt  to  be  fissured  and  filled  with  dikes,  and  in  many  cases,  if 
stratified,  with  intrusive  sheets.  The  significance  of  volcanic  necks 
has  been  previously  explained,  page  215. 


THE   IGNEOUS   ROCKS 


319 


Stocks.  —  This  term  has  been  applied  to  large  bodies  of  intru- 
sive rock  which,  in  the  form  of  magma,  have  ascended  into  the 
upper  region  of  the  earth's  crust,  and  there  solidified.  They  have 
become  visible  only  by  extended  erosion,  and  usually  have  a  more  or 
less  circular  or  oval  ground  plan.  Their  outer  surface,  or  plane  of 
contact,  cuts  across  the  inclosing  rocks,  is  more  or  less  irregular, 
and  the  mass  may  widen  in  extent  as  it  descends.  Their  size  may 
be  from  a  few  hundred  yards  to  a  number  of  miles  in  diameter. 
Since  they  are  apt  to  form  protuberant  topographic  features  through 


Fig.  251.  —  Alesna  volcanic  neck,  Mt.  Taylor  region,  New  Mexico.     C.  E.  Dutton, 

U.  S.  Geol.  Surv. 

erosion,  they  are  sometimes,  especially  in  Great  Britain,  called 
bosses.  The  distinction  from  a  volcanic  neck  is  not  one  of  size  alone, 
though  necks  tend  to  be  smaller  than  stocks,  but  in  that  the  term 
"neck"  is  employed  only  when  there  is  evidence  that  extrusive  vol- 
canic activity  has  been  connected  with  it.  Some  stocks  were  doubt- 
less necks,  but  this  cannot  now  be  proved.  The  granite  hills  of  New 
England,  and  of  many  other  old  eroded  mountain  regions  are  often 
largely  composed  of  stocks,  or  bosses. 

Bathyliths.  —  This  word  is  used  in  a  general  way  to  designate 
huge  irregular  masses  of  igneous  rock,  which,  underlying  the  sedimen- 
tary and  so-called  metamorphic  ones,  or  sometimes  cutting  through 
them,  have  been  exposed  by  erosion.  They  are  seen  most  commonly  in 
the  oldest  exposed  areas  of  the  earth's  crust,  where  they  are  character- 
istically accompanied,  or  surrounded,  by  metamorphic  rocks,  as  in 


320  TEXT-BOOK  OF  GEOLOGY 

eastern  Canada  and  New  England,  or  in  mountainous  regions  where 
they  form  the  central  cores  of  masses  of  the  ranges,  as  in  parts  of 
the  Rocky  Mountains.  They  differ  chiefly  from  stocks  in  their 
much  greater  size,  as  they  are  in  some  cases  exposed  over  many 
thousands  of  square  miles  of  surface. 

Although  some  stocks  are  clearly  intrusive,  and  have  displaced  the  rocks 
whose  site  they  occupy,  the  mode  of  formation  of  others,  and  of  bathyliths, 
is  still  a  subject  of  speculation.  Some  have  held  that  they  have  attained  their 
position  by  melting  and  assimilating  the  previous  formation  and  thus  replac- 
ing it,  while  others  have  urged  the  view  that  it  has  been  ruptured,  uplifted, 
and  driven  out  by  the  invading  mass  of  magma,  and  then  eroded  away.  Vari- 
ous modifications  of  these  views  have  been  suggested,  and  while  geologic 
science  is  not  yet  in  a  position  to  pronounce  definitely  upon  their  correctness, 
it  seems  probable  that  no  one  set  process  will  explain  all  cases,  and  that 
at  different  times  and  places  diverse  agencies  have  operated. 

Extrusive  Igneous  Rocks.  —  The  modes  of  occurrence  of  the  ex- 
trusive igneous  rocks  have  already  been  described  in  connection 
with  volcanoes  and  extrusions  of  lava,  and  what  has  there  been 
said  in  regard  to  them  may  be  profitably  consulted  in  this  connec- 
tion, page  203.  For  the  sake  of  convenience  the  following  summary 
is  here  given.  There  are  two  kinds  of  extrusions  of  magma,  depend- 
ing on  the  quantity  and  activity  of  gases  contained  in  it;  the  quiet, 
in  which  it  outwells  as  a  liquid  and  solidifies  into  rock,  and  the  ex- 
plosive, in  which  it  is  more  or  less  violently  driven  into  the  air  and 
falls  in  the  form  of  solid  fragments. 

Quiet  Eruption:  Lava  Flows.  —  Magma  which  appears  at  the 
surface  and  outpours  is  known  as  lava.  When  solidified  it  is  com- 
monly spoken  of  as  a  lava  flow,  or  extrusive  sheet.  Usually  such 
outflows  are  in  connection  with  volcanoes,  the  extrusions  of  a  few 
volcanoes  being,  indeed,  wholly  of  this  nature,  like  some  of  those  in 
Hawaii,  but  generally  lava  flows  succeed,  or  alternate  with,  pro- 
jections of  fragmental  material. 

In  other  cases  it  has  been  thought  that  they  are  not  connected  with  volcanic 
eruptions,  but  have  taken  place  as  quiet  outwellings  from  numerous  fissures. 
This  has  sometimes  occurred  on  a  huge  scale,  as  in  the  Columbia  River  region 
of  the  northwestern  United  States,  in  western  India,  and  in  the  north  of  the 
British  Isles.  In  the  first  two  regions  the  repeated  lava  flows  are  thousands 
of  feet  in  depth,  and. cover  areas  of  from  100,000  to  possibly  200,000  square 
miles.  This  view  of  the  origin  of  the  Columbia  lavas  has,  however,  been  dis- 
puted, as  mentioned  on  page  221. 

Not  infrequently  sheets  of  lava  have  sunk  below  sea-level  and  been  covered 
by  deposits  or  they  have  originated  on  the  sea-floor  and  have  been  covered. 
Such  buried  extrusive  sheets  are  distinguished  from  intrusive  ones  by  the  fact 
that  they  have  not  altered,  changed,  or  baked  the  sediments  above  them,  and 


THE   IGNEOUS   ROCKS  321 

their  upper  surfaces  usually  show  the  structures  common  to  the  surface  of 
lavas,  sach  as  the  vesicular,  scoriaceous,  and  ropy  ones  described  previously. 

Explosive  Eruption :  Tuffs  and  Breccias.  —  When  magma  at- 
tains the  surface  in  the  canal  of  a  volcano,  it  may  give  rise  to  quiet 
flows  of  lavas  as  mentioned  above,  or,  if  its  viscosity  is  sufficient 
and  it  is  charged  with  vapors  under  great  tension,  it  will  give  rise 
to  explosive  activity,  and  the  material  will  be  projected  into  the  air 
to  fall  in  solid  fragmental  form,  as  already  described  under  vol- 
canoes, page  212.  Owing  to  the  expansion  of  contained  vapors, 
chiefly  steam,  the  projected  pieces  usually  have  a  more  or  less  vesic- 
ular structure,  and  vary  in  size  from  large  blocks  to  fine  dust. 
According  to  size  this  material  may  be  roughly  classified  as  follows: 
pieces  the  size  of  an  apple  and  upward  are  termed  volcanic  bombs; 
those  the  size  of  nuts,  lapilli;  of  the  size  of  small  peas,  or  shot, 
volcanic  ashes;  while  the  finest  is  volcanic  dust.  The  coarser  ma- 
terial, the  bombs,  ashes  and  lapilli,  falls  around  the  vent  and 
builds  up  the  cone;  the  lighter  ashes  and  dust,  carried  by  air  cur- 
rents, tend  to  fall  after  these,  and  at  greater  distances.  The  beds 
of  coarser  material  thus  produced  are  termed  volcanic  conglomerate, 
or,  more  commonly,  volcanic  breccia,  while  the  finer  material  is 
known  as  tuff.  See  pages  208  and  213. 

Volcanic  Tuff.  —  The  rock  formed  by  the  consolidating  of  volcanic  ashes 
and  dust  is  usually  light  in  weight,  sometimes  of  a  chalky  consistency,  some- 
times quite  hard  and  dense.  It  is  of  various  colors,  generally  of  light  shades. 
When  very  fine  and  compact  a  tuff  might  be  mistaken  for  the  rock  of  a  lava 
flow,  but  generally  examination  will  show  angular  fragments  in  it,  and  in  some 
cases  the  tuffs  contain  excellent  imprints  of  fossils;  of  leaves,  twigs,  etc.,  if 
the  ashes  have  fallen  on  land,  or  of  marine  organisms,  such  as  fishes,  etc., 
if  into  water.  If  the  ashes  fell  into  water  the  tuffs  may  be  well  stratified  and 
interbedded,  perhaps  with  shales  and  sandstones.  Or,  volcanic  ashes  may  be 
washed  down  into  water,  and,  mingled  perhaps  with  the  ordinary  products 
of  land  waste,  give  rise  to  a  well  bedded  sedimentary  series  of  strata.  It  is 
here  that  gradations  of  igneous  into  sedimentary  rocks  may  occur.  Volcanic 
tuff  was  formerly  called  volcanic  tufa,  but  it  is  now  customary  to  restrict  the 
word  tufa  to  deposits  from  aqueous  solution,  especially  those  of  a  calcareous 
nature.  See  page  167. 

Volcanic  Breccia.  —  This  has  a  base  of  tuff  more  or  less  filled  with  angular 
pieces  and  bombs,  and  masses  which  are  apt  to  be  rounded.  It  often  contains 
fragments  of  the  rocks  through  which  the  conduit  has  been  drilled.  These 
characters  distinguish  breccias,  even  when  they  are  very  hard  and  much 
changed  by  certain  processes  which  act  upon  rocks.  Usually,  they  are  rather 
soft  and  easily  attacked  by  erosion ;  the  cement,  or  base,  goes  first,  leaving  the 
bombs  and  contained  masses  projecting.  In  this  way  on  the  edges  of  cliffs 
oddly  shaped  figures  of  erosion  are  produced,  in  regions  where  volcanic  breccias 
are  common,  as  in  parts  of  the  Rocky  Mountains. 


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Tuffs  and  breccias  are  rocks  of  wide  distribution,  being  found  in  all  those  dis- 
tricts where  volcanic  activity  is  being,  or  has  been,  displayed,  and  their  pres- 
ence is,  indeed,  one  of  the  surest  indications  of  its  occurrence  in  former  times 
in  places  where  vulcanism  has  long  since  died  out.  We  are  thus  able  to 
recognize  the  former  existence  of  volcanoes  in  various  parts  of  the  eastern 
United  States  and  Canada,  from  Nova  Scotia  to  Georgia.  In  the  regions 
of  the  Rocky  Mountains  they  are  found  in  vast  quantities,  piled  up  in 
places  thousands  of  feet  in  thickness,  where,  as  in  western  Wyoming,  serried 
mountain  peaks  have  been  cut  from  them  by  erosion. 


Fig.  252.  —  Diagram  to  illustrate  the  occurrence  of  igneous  rocks:  6,  bathylith; 
s,  stock;  n,  volcanic  neck  forming  v,  a  volcano  with  tuffs  and  breccias;  I,  I,  lacco-r 
liths;  i,  intrusive  sheet;  e,  extrusive  sheet;  d,  d,  dikes.  Horizontal  distance 
shown  thirty  miles;  vertical  distance,  three  miles. 

Age  of  Igneous  Rocks.  —  The  determination  of  the  geologic 
period,  when  a  given  mass  of  igneous  rock  was  erupted,  or  intruded, 
is  made  by  observing  its  relations  to  the  previously  existent  rocks 
with  which  it  has  come  in  contact.  Thus,  if  one  body  of  igneous 
rock,  such  as  a  dike,  passes  through,  or  cuts,  another  body  of  rock, 
it  is  the  younger  of  the  two.  If  it  cuts  across  stratified  beds  it  is 
younger  than  they  are,  and  lavas,  of  course,  are  more  recent  than  the 
rocks  upon  which  they  lie.  If  a  sheet  of  igneous  rock  lying  con- 
cordantly  between  strata  has  affected  the  beds  both  above  and 
below  (see  contact  metamorphism,  page  350),  it  is  younger  than 
both.  If  the  overlying  beds  are  not  changed  the  sheet  may  be  a 
lava  flow,  see  page  315,  and  older  than  they  are.  It  is  thus  usually 
easy  to  tell  when  an  igneous  mass  is  younger  than  other  rocks  by  ex- 
amining its  contacts  with  them,  but  much  more  difficult  to  say  when 
it  is  older  than  they  are,  because,  usually;  the  younger  beds  have 
been  eroded  away  if  the  mass  is  exposed,  or  they  still  conceal  it,  or 
it  may  never  have  been  covered.  The  age  of  the  stratified  rocks  is, 
of  course,  determined  by  their  fossils,  and  the  endeavor  is  made  to 
bring  the  igneous  ones,  which  contain  no  fossils,  into  time  relation 
with  them. 


Kinds  and  Classification  of  Igneous  Rocks 

Introductory.  —  Those  features  of  igneous  rocks,  by  which  the 
different  kinds  are  distinguished,  have  been  already  mentioned  in  a 


THE   IGNEOUS   ROCKS-  323 

preliminary  way  in  the  discussion  of  the  products  of  volcanoes, 
page  203,  but  should  now  receive  the  attention  which  they  demand 
in  this  place.  Igneous  rocks  are  divided  into  different  kinds  on  the 
basis  of  two  properties,  first,  their  composition,  and  second,  their 
minute  structure,  or  texture.  Each  of  these  requires  explanation. 

Composition  of  Igneous  Rocks.  —  Since  igneous  rocks  are  pro- 
duced by  the  solidification  of  magmas  it  is  evident  that  their  com- 
position will  depend  on  the  chemical  composition  of  these  molten 
fluids.  It  has  been  already  shown,  page  201,  that  a  magma  con- 
sists of  two  parts,  volatile  constituents,  which  are  given  off  as  water 
vapor,  carbon  dioxide,  sulphur  fumes,  etc.,  and  a  non-volatile  part, 
a  molten  flux,  consisting  chiefly  of  the  oxides  of  certain  metals  and 
silica.  Although  the  gaseous  part  is  important,  for  reasons  we  can- 
not now  consider,  in  rock  formation,  it  is  essentially  the  melted 
oxides  which  give  rise  to  the  rocks  and  are  the  chief  and  determining 
constituents  of  magmas.  While  it  is  evident  that  a  molten  magma 
cannot  be  analyzed  directly  by  chemical  methods,  still,  the  kinds 
of  oxides  present,  and  their  relative  proportions,  can  be  ascertained 
by  analysis  of  the  cold  and  solid  rock.  This  has  been  done  with 
many  thousands  of  igneous  rocks  from  all  parts  of  the  world  and, 
in  a  general  and  rudely  approximate  way,  the  following  results  have 
been  obtained,  which  show  the  composition  of  the  magmas. 

Silica,  SiO2,  always  present;  may  vary  from  35  to  75  per  cent. 

Alumina,  A12O3,  varies  from  nothing  to  25  per  cent. 

Oxides  of  iron,  FeO  and  Fe2O3,  usually  both,  0-20  per  cent. 

Magnesia,  MgO,  0-45  per  cent. 

Lime,  CaO,  0-20  per  cent. 

Soda,  Na2O,  0-16  per  cent. 

Potash,  K20,  0-12  per  cent. 

It  must  not  be  concluded  from  inspection  of  the  above  table  that  within  the 
limits  given,  any  and  all  sorts  of  mixtures  of  these  oxides  can  occur.  As  we 
shall  see  presently,  there  are  certain  general  laws  governing  their  associations. 
It  will  also  be  noticed  that  there  is  one  definite  acid-forming  oxide,  silica, 
present,  while  the  oxides  of  the  six  metals,  aluminum,  iron,  magnesium,  cal- 
cium, sodium  and  potassium,  are,  in  general,  bases.  Since  we  have  then  an 
acid  and  bases  in  the  magma,  according  to  one  of  the  fundamental  principles 
of  chemistry,  there  will  be  opportunity,  under  suitable  conditions,  for  the 
formation  of  salts.  What  these  salts  are  we  shall  learn  later.  Oxides  of 
other  elements  occur  in  small  or  minute  quantities,  but  are  of  so  much  less 
importance  that  they  may  be  neglected. 

Associations  of  Oxides  in  Magmas.  —  Although  there  are  many 
exceptions  to  this  rule,  it  has  been  found  to  be  generally  true  that 
large  percentages  of  potash  and  soda  (alkalies)  in  a  magma  are 


324  TEXT-BOOK  OF  GEOLOGY 

accompanied  by  correspondingly  large  ones  of  alumina  and  silica 
and  by  consequent  small  amounts  of  the  other  three  metallic 
oxides.  Conversely,  large  percentages  of  magnesia,  lime  and  iron 
oxides  are  apt  to  be  associated,  and  these  go  with  low  silica;  the 
alumina  and  alkalies  being  small,  or  wanting.  These  reciprocal 
relations  are  of  general  fundamental  importance  in  igneous  rocks, 
and  it  will  be  recalled  that  they  have  been  pointed  out  before,  page 
201,  since  the  nature  of  volcanic  activity,  and  the  kinds  of 
volcanoes,  in  large  measure  depend  upon  them.  They  may  be  ex- 
pressed in  a  general  way  as  follows : 

Where  Si02,  A12O3,  (Na,K)2O  are  high,  CaO,  MgO,  FeO  are  low  or  wanting. 
Where  CaO,  MgO,  FeO  are  high,  SiO2  is  low,  and  A12O3,  (Na,K)2O  are  low 
or  wanting. 

Crystallization.  —  It  is  a  familiar  chemical  experiment  that,  if 
one  places  zinc  in  sulphuric  acid  diluted  with  water,  it  will  quickly 
disappear  with  evolution  of  hydrogen,  and,  in  the  place  of  the  acid 
and  the  base  (metal) ,  the  vessel  will  contain  a  salt,  zinc  sulphate,  in 
solution.  If  the  liquid  be  boiled  down  and  concentrated  to  a  certain 
point,  the  zinc  sulphate  can  no  longer  remain  in  solution,  but  will 
begin  to  appear  in  solid  condition  in  the  form  of  crystals.  If  the 
hot  solution  be  allowed  to  cool,  more  crystals  of  the  salt  will  be 
formed,  since,  in  general,  hot  solutions  can  contain  more  salt  than 
cold  ones.  In  analogy  with  what  has  been  stated  above,  a  molten 
magma  is  to  be  regarded  as  of  the  nature  of  a  solution ;  it  contains 
silica,  an  acid-forming  oxide,  and  metallic  oxides,  which  are  bases. 
If  the  magma  cools  with  sufficient  slowness  these  will  unite  to  form 
salts,  which  at  proper  temperatures  will  take  on  solid  form  and 
appear  as  crystals.  This  will  proceed  as  the  temperature  falls  until 
the  whole  magma  turns  into  a  mass  of  solid  crystal  grains.  The 
molten  liquid  has  become  stone.  It  rarely  happens  that  the  propor- 
tions of  acid  and  bases  are  so  exactly  balanced  in  a  magma,  or  the 
circumstances  are  such,  that  it  is  completely  turned  into  salts;  nearly 
always  there  is  an  excess  of  the  acid,  which  appears  as  solid  silica, 
Si02  (quartz),  or  of  some  of  the  metallic  oxides  (iron  oxides,  alu- 
mina, etc.) ,  which  latter  may  combine.  Such  solid  compounds,  salts 
and  oxides,  occurring  in  nature  are  minerals,  and  we  see  from  this 
that  igneous  rocks  are,  in  general,  composed  of  mineral  grains  of 
various  kinds,  and  that  these  grains  have  crystallized  from  the 
magma. 

Kinds  of  Minerals.  —  The  more  important  of  the  minerals  which 
form  the  igneous  rocks  are  the  following: 


THE   IGNEOUS   ROCKS  325 

Feldspar  Group  Ferromagnesian  Group 

Orthoclase  Feldspar,  KAlSi3Og  Mica  (Biotite),  K2  (Mg,Fe)2  AlgSi  O 

Albite  Feldspar,  NaAlSi3Og  Pyroxene,   Ca(Mg,Fe)Si206 

Anorthite  Feldspar,  CaAl2Si2Og  Hornblende,  Ca(Mg,Fe)3Si4O 

Nephelite,  NaAlSiO4  Olivine,   (Mg,Fe)2SiO4 

Also  Quartz,  Si02  Magnetite  (Iron  Ore),  Fe3O4 

Of  these  minerals,  feldspars,  quartz,  pyroxene,  and  hornblende  are 
the  most  important  in  forming  igneous  rocks  and  the  student 
should,  therefore,  make  careful  note  of  them.  For  further  details 
regarding  them  reference  should  be  had  to  the  Appendix  dealing  with 
the  minerals  mentioned  in  this  work.  It  will  be  seen  by  examining 
their  chemical  formulas,  as  given  in  the  table  above,  that  they  are 
composed  of  silica  and  the  six  metallic  oxides  previously  mentioned 
as  forming  the  magmas. 

Furthermore,  since  it  was  shown  that  the  relative  proportions 
of  the  oxides  varied  in  the  magmas,  it  is  evident  that  the  relative 
quantities  of  the  minerals  will  also  vary.  Thus,  a  magma  in  which 
(Na,K)20,  A1203  and  Si02  are  the  chief  substances  will  form  a  rock 
consisting  mostly  of  feldspars,  while  one  in  which  CaO,  MgO  and 
FeO  are  high  will  make  rocks  containing  largely  or  mostly  pyroxene, 
hornblende  and  other  jerromagnesian  minerals,  as  they  are  called  in 
allusion  to  the  iron  and  magnesia  in  them,  or  mixtures  of  these 
minerals. 

Thus  it  appears  that  igneous  rocks  vary  in  the  kinds  and  relative 
amounts  of  the  minerals  which  compose  them,  and  on  these  varia- 
tions, as  we  shall  see  later,  depend  mainly  the  different  varieties  of 
igneous  rocks,  and  the  manner  of  classifying  them. 

Texture.  —  While  igneous  rocks  are  distinguished  and  classified 
in  one  way  by  the  kinds  of  mineral  grains  which  compose  them, 
they  are  also  classified  in  another  way  by  the  textures  which  they 
may  exhibit.  By  texture  is  meant  the  relative  size,  or  sizes,  of  the 
component  grains  and  their  relation  to  each  other.  Thus,  if  the 
grains  were  as  large  as  peas  we  should  say  that  in  texture  such  a 
rock  was  coarse-grained;  if  like  ordinary  loaf  sugar,  such  a  rock 
would  be  fine-grained;  while,  if  the  particles  were  so  fine  that  they 
could  not  be  discriminated  by  the  eye  and  the  rock  appears  like  a 
homogeneous  substance,  we  should  say  it  was  compact,  or  dense,  in 
texture. 

This  evidently  depends  on  the  size  of  the  particles,  or  crystal 
grains,  and  that  in  turn  depends  on  the  rate  at  which  the  magma 
cooled.  For,  if  the  magma  is  too  hot,  as  explained  above  under 
crystallization,  this  process  cannot  take  place,  and  no  crystals  will 


326 


TEXT-BOOK   OF   GEOLOGY 


begin  to  form  until  the  proper  temperature  is  reached;  then  they 
will  commence  to  appear  and,  if  the  cooling  is  very  slow,  they  will 
have  time  to  grow  to  large  size,  giving  a  coarse-grained  rock.  But, 
if  the  cooling  is  rapid,  new  centers  of  crystallization  will  be  more 
and  more  forced  to  form,  and,  if  the  process  is  thus  hurried,  instead 
of  fewer  crystals  growing  to  larger  sizes,  the  rock  will  consist  of 
a  much  greater  number  of  smaller  particles,  or  will  be  fine-grained 
in  texture.  And  with  still  more  rapid  cooling  the  particles  may  be 
so  minute  that  the  rock  has  the  dense  texture.  Analogy  will  now 


Fig.  253.  —  A.    Even-granular  Rock.  B.   Porphyry. 

carry  us  one  step  more;  we  could  conceive  that  the  cooling  might 
take  place  with  such  great  rapidity  that  the  magma  would  solidify 
into  a  homogeneous  substance  before  any  crystallization,  which 
consists  in  the  molecules  arranging  themselves  together  to  form 
definite  solid  compounds,  could  occur.  In  this  event  we  should  have 
glass,  or  a  glassy  rather  than  a  stony  texture,  as  the  result,  a  case 
which  sometimes  happens. 

To  sum  up,  then,  we  see  that  igneous  rocks  in  their  texture  may 
be  coarse-grained,  fine-grained,  dense,  or  glassy,  and  that  this  de- 
pends on  the  rate  of  cooling  of  the  magma. 

Porphyry :  Porphyritic  Texture.  —  In  what  has  been  said  so  far 
regarding  the  texture  of  igneous  rocks,  it  has  been  tacitly  assumed 
that  the  component  mineral  grains  are,  in  any  given  rock,  of  uni- 
form size,  or  that  the  rock  is  evenly  granular,  as  it  is  called.  This 
is,  however,  by  no  means  always  the  case.  For  inspection  of  igneous 
rocks  shows  that  in  many  cases  they  are  composed  of  crystals  of 


THE   IGNEOUS   ROCKS  327 

two  sizes;  some  larger  and  more  distinct  embedded  in  a  matrix  of 
much  finer  grains.  An  igneous  rock  having  this  latter  texture  is 
called  a  porphyry.  Examples  of  the  even-granular  and  porphyritic 
textures  are  seen  in  Fig.  253,  A  and  B.  The  matrix  of  a  porphyry 
is  termed  the  ground-mass,  and  the  larger  embedded  grains,  or 
crystals,  the  phenocrysts  (evident  crystals).  The  porphyries  are 
a  large  and  important  division  of  igneous  rocks. 

The  ground-mass  (matrix)  may  itself  vary  greatly  in  texture:  it  may  be 
medium-grained,  fine-grained,  compact,  or  glassy;  most  commonly  it  is  fine- 
grained, or  compact.  And  the  phenocrysts  may  also  vary  widely ;  they  may  be 
of  large  size,  as  large  as  walnuts,  or  as  small  as  grains  of  sand;  they  may  be 
abundant,  or  comparatively  few.  But  always  there  is  this  contrast  between 
sizes  of  crystals,  between  ground-mass  and  phenocrysts,  which  makes  the 
essence  of  a  porphyry,  and  the  student  should  guard  against  making  it  a  con- 
trast of  colors;  thus  a  rock  consisting  of  grains  of  light  colored  quartz  and 
feldspar,  in  which  are  embedded  a  few  black  crystals  of  mica,  all  grains  being 
of  about  the  same  size,  is  not  a  porphyry. 

Relation  of  Texture  to  Geologic  Mode  of  Occurrence.  —  Since, 
as  has  been  shown  above,  the  texture  of  an  igneous  rock  depends 
chiefly  upon  the  rate  at  which  the  magma  cools,  it  is  clear  that  this 
in  turn  will  depend  most  largely  upon  its  final  resting  place.  For 
it  is  obvious  that  an  intrusive  mass  of  magma,  surrounded  and 
blanketed  above  by  other,  older  rock  masses,  must  lose  heat  much 
more  slowly  than  an  extrusive  one,  poured  out  on  the  surface  in  the 
form  of  lava.  Hence,  as  coarse-grained  textures  mean  slow  cooling, 
we  naturally  associate  them  with  intrusive  masses,  and,  conversely, 
regard  the  dense,  or  glassy  types  as  belonging  to  the  products  of 
extrusion,  the  surface  lava  flows.  But  it  is  also  clear  that  the  size 
of  the  intruded  mass  will  have  much  to  do  with  the  rate  of  cooling, 
since  a  very  large  mass  cools  more  slowly  than  a  small  one.  Thus, 
the  great  bathyliths  and  stocks  are  naturally  coarse  in  texture, 
whereas  dikes  and  intruded  sheets  tend  to  be  much  finer,  or  compact. 
On  the  other  hand,  it  is  easy  to  conceive  that  the  central  portion 
of  an  extremely  thick  lava  flow  might  cool  with  sufficient  slowness  to 
afford  a  medium-grained  type  of  texture,  while  a  magma  forced 
into  a  narrow  fissure  in  cold  rocks  might  be  chilled  so  quickly  as  to 
assume  a  dense,  compact,  or  even  glassy  one.  Thus  various  modi- 
fications of  the  general  rule  can  be  easily  imagined,  according  to 
particular  cases,  and  yet  this  general  rule,  that  the  intrusive  rocks 
are  medium-  to  coarse-grained,  the  extrusive  ones  fine-grained  to 
dense,  is  nevertheless  true. 

An  important  deduction,  which  follows  from  the  above,  is  that  the 


328  TEXT-BOOK  OF  GEOLOGY 

coarse-grained  rocks,  since  they  have  been  formed  in  depth,  can 
only  become  visible  at  the  surface  after  a  prolonged  period  of  ero- 
sion, which  has  been  sufficient  to  remove  the  cover  and  expose  the 
igneous  mass. 

With  respect  to  what  has  just  been  stated,  the  porphyries  occupy  a 
somewhat  intermediate  position.  It  would  carry  us  too  far  to  dis- 
cuss in  this  place  the  most  probable  causes  for  this  type  of  texture, 
but,  in  general,  one  may  say  the  porphyritic  texture  is  evidence 
of  a  rather  rapid  and  variable  rate  of  cooling  and  crystallization. 
Consequently,  it  is  an  infrequent  feature  in  the  great  stocks  and 
bathyliths,  is  much  more  common  in  the  smaller  intrusions,  such  as 
laccoliths,  dikes  and  intrusive  sheets,  and  is  very  commonly  found 
in  lava  flows. 

While  the  rate  of  cooling  is  the  most  important  factor  influencing  the  tex- 
ture of  igneous  rocks,  as  discussed  above,  it  is  not  the  only  one.  The  subject 
is  too  complicated  a  matter  for  detailed  treatment  in  this  work,  but  it  may  be 
mentioned  that  the  chemical  composition  also  has  its  influence;  those  magmas 
with  low  silica  and  much  iron  and  magnesia  tending  to  assume  coarser  grain 
than  the  ones  composed  of  much  silica,  alumina  and  alkalies,  under  similar 
conditions  of  cooling.  The  reason  appears  to  be  that  the  former  are  less 
viscous,  or  more  fluid,  at  lower  temperatures  than  the  latter,  as  already  ex- 
plained, page  201,  and  thus  when  they  crystallize  this  mobility  of  the  mole- 
cules permits  the  growth  of  larger  crystal  grains. 

Also  the  presence  of  the  included  vapors  which  magmas  contain,  especially 
water  vapor,  increases  the  fluidity  and  promotes  a  coarser  crystallization. 
This  is  very  notably  shown  in  certain  places  in  intrusive  masses,  in  cracks  and 
fissures  which  have  served  as  channels  of  escape  for  the  gases,  and  which  are 
now  filled  with  large  and  even  huge  crystals  of  quartz,  feldspar  and  mica. 
These  very  coarse  masses  are  known  as  pegmatites,  and  from  them  are  ob- 
tained the  plates  of  mica  which  are  used  commercially. 

In  the  case  of  a  volcanic  neck,  the  rock  is  apt  to  be  comparatively  coarse- 
textured  for  a  small  mass,  because,  by  the  constant  upward  passage  of  molten 
material  to  the  surface,  the  rock  masses  surrounding  the  channel  have  become 
greatly  heated,  thus  producing  slow  cooling  in  the  last  charge  of  magma  which 
occupies  the  conduit  and  solidifies  there  when  the  volcano  becomes  extinct. 

Classification  of  Igneous  Rocks.  —  The  different  features  by 
which  the  igneous  rocks  may  be  classified  have  now  been  explained. 
We  have  seen  that  they  vary  in  the  kinds  and  relative  amounts  of 
the  mineral  grains  composing  them,  and  that  they  also  vary  in 
texture.  Both  features  are  used  to  classify.  Thus  we  may  at 
once  divide  the  igneous  rocks  into  two  groups  on  the  basis  of  texture ; 
one  in  which  the  mineral  grains  are  sufficiently  large  to  be  identified 
by  the  eye  alone,  or  aided  by  a  pocket  lens;  and  another  in  which 
they  are  too  minute  for  this  to  be  done.  In  the  latter  case  further 


THE    IGNEOUS    ROCKS 


329 


subdivision  has  to  be  carried  out  on  the  basis  of  color  and  general 
appearance  and  according  to  this  the  rocks  are  divided  into  felsites 
(light  or  medium  colored  rocks)  and  basalt  (black  or  nearly  black 
rocks).  These  have  been  previously  explained.  The  group  in 
which  the  grains  are  sufficiently  large  for  the  component  minerals 
to  be  recognized  (usually  about  the  texture  of  loaf  sugar)  may  be 
termed  grained  rocks  and  can  be  subdivided  on  the  proportions  of 
the  constituent  minerals.  They  can  be  divided  into  two  main 
groups,  one  in  which  feldspar  is  the  predominant  substance;  and 
another  in  which  it  is  subordinate,  or  lacking,  and  the  dark  iron  and 
magnesia  minerals  (hornblende  and  pyroxene)  are  the  chief  com- 
ponents. Each  of  these  may  be  further  subdivided,  in  the  first, 
as  to  whether  the  feldspar  is  accompanied  by  quartz,  or  not,  and  in 
the  second,  as  to  whether  pyroxene  or  hornblende  is  the  dominant 
ferromagnesian  mineral.  Further  divisions  are  made  as  to  whether 
the  texture  is  even-granular  or  porphyritic.  These  various  dis- 
tinctions and  the  names  of  the  different  kinds  of  igneous  rocks  which 
they  make  are  exhibited  in  the  following  table. 

TABLE  OF  CLASSIFICATION  OF  IGNEOUS  ROCKS 
A.  Grained,  constituent  grains  recognizable.     Mostly  intrusive 


a.   Feldspathic  rocks,  usually 
light  in  color 

6.   Ferromagnesian  rocks,  generally 
dark  to  black 

With  quartz 

Without  quartz 

With  subordinate 
feldspar 

Without  feldspar 

Even-granular 
(nonporphyritic) 

GRANITE 

SYENITE 

DIORITE 

(with  hornblende) 

GABBRO 
(with  pyroxene) 

PERIDOTITE 
Pyroxenite 
Hornblendite 

DOLERITE 

(undetermined) 

Porphyritic 

GRANITE- 
PORPHYRY 

SYENITE- 
PORPHYRY 

DIORITE-.ETC., 
PORPHYRY 

B.  Dense,  constituent  grains  nearly,  or  wholly,  unrecognizable.    Intrusive  and  extrusive 


a.   Light  colored,  variable; 
usually  feldspathic 

b.   Dark  colored  to  black;  usually 
ferromagnesian 

Nonporphyritic 
Porphyritic 

FELSITE 
FELSITE-PORPHYRY 

BASALT 
BASALT-PORPHYRY 

C.   Rocks  composed  wholly,  or  in  part,  of  glass,     Extrusive 


Nonporphyritic 
Porphyritic 


OBSIDIAN,  Pitchstone,  Pumice,  Scoria,  etc. 
Vitrophyre  (Glass-porphyry) 


D.   Fragmental  igneous  material.     Extrusive 


TUFF  and  BRECCIA  (Volcanic  ashes,  bombs,  etc.) 


330 


TEXT-BOOK   OF   GEOLOGY 


Remarks  on  the  Table.  —  Leaving  out  of  account  the  rare  glassy  rocks, 
and  tuff  and  breccia,  the  following  remarks  may  prove  of  service  to  the 
student  in  understanding  the  classification  of  igneous  rocks,  as  shown  by 
the  table  given  above,  especially  to  those  who  have  little,  or  no  knowledge 
of  mineralogy. 

He  should  notice  that  there  are  three  textures  employed:  grained  (rela- 
tively coarse) ,  dense,  and  porphyritic.  Since,  ordinarily,  he  cannot  be  expected 
to  distinguish  between  pyroxene  and  hornblende,  the  only  distinctions 
which  affect  the  mineral  composition  are  based  on  color,  light  to  medium, 
or  dark  to  black,  with  one  exception.  The  exception  is  the  difference 


Fig.  254.  —  Thin  section  of  a  rock. 

between  quartz  and  feldspar,  which  shows  whether  the  former  is  present,  or 
not,  and  this  is  based  on  cleavage.  Feldspar  has  good  distinct  cleavage, 
quartz  has  none;  see  Appendix  A,  on  cleavage,  feldspar,  and  quartz. 

As  a  result  of  these  differences  of  texture,  color,  and  cleavage,  the  student 
will  observe  that  the  igneous  rocks  form  six  groups  as  follows :  granite,  syenite, 
dolerite,  peridotite,  jelsite  and  basalt,  and  each  of  these,  with  the  proper  tex- 
ture, is  a  corresponding  porphyry.  The  first  four  are  grained  rocks,  distin- 
guished by  minerals  and  color,  the  last  two  are  dense,  and  are  told  by  the  color 
alone.  Peridotite  porphyry  is  known,  but  is  so  rare  that  its  mention  is 
omitted.  With  these  points  in  mind  the  classification  is  simple. 

Method  of  Study.  —  The  classification  which  has  just  been  described  is 
based  upon  what  can  be  recognized  by  the  eye,  aided,  perhaps,  by  a  pocket 
lens.  It  is  thus  termed  a  field  classification  and  sometimes  megascopic  (Greek 
mega,  great),  in  contrast  to  one  based  upon  results  obtained  microscopically, 
by  the  study  of  thin  rock  slices.  In  the  latter  method  a  chip  of  rock  is 
ground  flat  with  emery  on  a  metal  plate,  then  cemented  to  a  piece  of  glass 
and  the  other  side  ground  down,  first  with  coarse,  and  successively  with  finer 
and  finer  emery  powder  until  the  section  is  as  thin  as  paper.  In  this  way 
the  minute  mineral  grains  composing  the  densest  and  blackest  of  basalts 
become  transparent,  and  may  be  studied  and  determined  under  the  micro- 
scope. See  Fig.  254.  In  this  study  polarized  light  is  used,  and  a  general 
knowledge  of  minerals,  of  their  crystal  characters  and  optical  properties,  is 
necessary.  It  would  require  too  much  detail  to  describe  further  this  mode 
of  studying  rocks,  which  combined  with  the  examination  of  them  by  chemical 
means,  has  developed  into  a  separate  geological  science,  called  Petrology, 
the  science  of  rocks,  but  it  should  be  stated  that  so  much  additional  informa- 


THE   IGNEOUS   ROCKS  331 

tion  is  gained  by  these  processes  that  the  ultimate  classification  of  igneous 
rocks  is  much  more  complicated  than  the  simple  scheme  outlined  above. 

Granite.  —  As  may  be  seen  from  the  scheme  of  classification, 
this  rock  is  composed  chiefly  of  quartz  and  feldspar.  It  also  gen- 
erally contains  in  variable  amount  dark  specks  or  flakes  of  mica,  less 
commonly  of  hornblende,  or  both.  It  is  the  most  important  intru- 
sive igneous  rock,  and  appears  to  be  one,  if  not  the  main,  constituent 
of  the  basement  part  of  the  continental  masses,  or  floor  upon  which 
the  rocks  of  later  sedimentary  age  were  deposited.  It  also  appears 
of  younger  age  in  the  form  of  great  stocks  and  vast  bathyliths 
which  have  displaced  these  younger  rocks.  In  these  occurrences  it 
is  either  in  the  form  of  true  granite,  or  in  a  certain  modification  of  it 
which  we  shall  see  later  among  the  metamorphic  rocks,  and  known 
as  gneiss.  Since  granite  is  formed  at  some  depth  it  is  only  after  pro- 
longed erosion  that  it  appears  at  the  surface,  hence  it  is  seen  chiefly 
in  those  parts  of  the  continents  long  exposed,  and  especially  where 
crumpling  and  crushing  of  the  shell  has  happened ;  that  is  to  say,  in 
mountain  regions.  There  are  many  varieties  of  granite,  based  on 
color,  texture,  etc.  Its  common  occurrence  is  shown  in  the  fact 
that  there  are  few  states  in  the  Union,  or  provinces  in  Canada, 
which  do  not  contain  exposures  of  granite,  and  its  use  as  a  building- 
stone,  and  for  various  purposes,  is  well  known. 

Syenite.  —  This  is  like  granite  in  composition  and  texture,  but  differs  in  con- 
taining little,  or  no  quartz.  Several  varieties  are  distinguished,  based  on  the 
character  of  the  feldspar,  and  the  accompanying  feldspar-like  mineral.  Thus  in 
syenite  proper  the  feldspars  are  alkalic,  that  is,  contain  soda  and  potash,  but 
little,  or  no  lime.  In  another  variety,  in  addition  to  the  feldspars,  a  feldspar- 
like  mineral,  nephelite,  NaAlSiO4,  is  present,  and  the  rock  is  known  as 
nephelite-syenite,  or  foyaite.  Another  rare  type  contains  corundum,  A12O3. 
These  syenites  are  not  common  rocks,  nor,  as  a  rule,  do  they  occur  in  very 
large  masses  compared  with  granite.  A  variety  of  this  class  of  igneous  rocks, 
one  in  which  the  feldspar  known  as  labradorite,  composed  of  soda  and  lime, 
is  the  chief  constituent,  is  called  anorthosite.  It  is  of  importance,  although 
the  number  of  known  occurrences  is  not  many,  from  the  large  and  sometimes 
vast  masses  which  it  forms.  Areas  of  it  are  found  in  eastern  Canada,  from 
Labrador  to  Ontario,  covering  hundreds  and  even  thousands  of  square  miles; 
in  the  Adirondacks,  Minnesota,  Norway,  etc. 

Diorite,  Gabbro,  Dolerite.  —  These  are  usually  more  or  less  dark 
colored,  heavy,  massive  rocks.  The  iron-magnesia  silicate,  in  the 
coarser  varieties,  forms  dark  to  black  specks  and  grains  which 
equal  in  number,  or  overbalance,  the  lighter  colored  grains  of 
feldspar.  The  kinds  are  based  on  the  nature  of  the  ferromagnesian 
mineral;  if  it  is  hornblende  the  rock  is  diorite,  if  pyroxene  it  is 


332  TEXT-BOOK  OF  GEOLOGY 

called  gabbro.  The  distinction  between  the  two  ferromagnesian 
minerals,  given  in  the  Appendix,  is  often  very  difficult  to  make,  and 
not  infrequently  impossible.  In  this  case  the  term  dolerite  may  be 
used,  signifying  a  rock  with  an  undetermined  ferromagnesian  com- 
ponent equal  to,  or  in  excess  of,  the  feldspar.  This  class  of  rocks, 
while  very  abundant  in  intrusions,  is  not  commonly  found  in  the 
extensive  bathyliths  and  stocks  in  which  granite  occurs.  They  are 
more  often  seen  in  smaller  stocks,  intrusive  sheets,  dikes,  and  some- 
times forming  the  inner  part  of  heavy  extrusive  masses.  When 
rather  fine-grained,  appearing  as  dark  heavy  rocks,  occurring  in 
dikes  and  sheets,  they  are  sometimes  called  trap,  from  a  Germanic 
word  meaning  stairs,  in  allusion  to  the  step-like  appearance  the 
exposed  dikes  often  present. 

There  are  many  varieties  of  this  class  of  rocks  recognized  by  petrologists, 
based  on  the  presence  of  particular  minerals  and  textures.  One  of  the  most 
important,  composed  of  pyroxene,  labradorite  feldspar,  and  iron-ore,  with  a 
certain  textural  arrangement  of  the  minerals,  is  known  as  diabase.  The  larger 
part  of  the  great  trap  sheets  of  the  lower  Connecticut  valley  and  of  northern 
New  Jersey  is  composed  of  diabase. 

Peridotites.  —  These  rocks,  composed  generally  of  variable  mixtures  of  fer- 
romagnesian minerals,  with  olivine  (peridote),  (Mg,Fe)2SiO4,  are  not  common, 
and  are  usually  found  in  minor  intrusions,  dikes,  sheets,  small  stocks,  etc. 
They  are  very  interesting  and  important,  however,  as  being  the  source  of 
ores  of  chromium,  nickel,  platinum,  and  of  the  diamond.  They  are  generally 
very  dark  to  black  and  heavy  from  the  large  amount  of  iron-bearing  minerals 
present.  A  variety  composed  wholly  of  olivine  is  known  as  dunite,  and  in  the 
Carolinas  the  occurrences  of  it  contain  the  mineral  corundum,  A1203,  which  is 
used  as  an  important  abrasive  in  the  manufacture  of  emery  wheels,  etc.  The 
diamonds  of  South  Africa  occur  in  volcanic  necks,  or  pipes,  composed  of 
this  rock,  and  they  have  been  also  found  in  somewhat  similar  intrusions  of  it 
in  Arkansas. 

Porphyries.  —  As  may  be  seen  by  reference  to  the  table  of  classi- 
fication there  are  various  kinds  of  porphyry,  depending  on  the 
coarseness  of  the  ground-mass  and  its  composition,  and  on  the  kinds 
of  minerals  which  may  be  embedded  in  it  as  phenocrysts.  Thus 
we  may  have  granite-porphyry,  syenite-porphyry,  or  felsite-por- 
phyry.  Feldspars  are  the  most  common  phenocrysts,  quartz  is 
seen  in  many  occurrences ;  sometimes  dark  to  black  flakes  or  prisms 
of  mica,  hornblende  or  pyroxene  occur.  The  porphyries  are  a  very 
common  class  of  rocks,  found  chiefly  in  the  minor  intrusions,  in 
dikes,  sheets,  and  laccoliths,  and  often  in  necks ;  they  are  not  often 
seen  in  the  great  stocks  and  bathyliths.  They  are  seen  also  as  com- 
posing many  extrusive  lava  flows.  The  intrusions  of  porphyry  in 
the  Rocky  Mountains'  region  are  very  common,  and  in  many  cases 


THE   IGNEOUS   ROCKS 


333 


are  accompanied  by  valuable  deposits  of  gold,  silver,  lead,  and  other 
ores,  where  they  come  in  contact  with  limestone.  Examples  of  this 
are  seen  at  Leadville  and  other  places  in  Colorado,  Montana, 
Nevada,  etc.  Porphyries  rarely  make  good  building  stones,  as  the 
masses  are  generally  too  much  divided  by  joints,  but  in  places  they 
serve  as  excellent  road  material.  A  porphyry  is  shown  in  Fig.  253. 
Felsite  and  Basalt.  —  Felsite  represents  the  dense  lava  forms  of 
the  intrusive  granites,  syenites,  etc.  Pure  felsite  is  not  common 
because,  generally,  phenocrysts  of  feldspar,  quartz,  and  mica  are 
present,  making  felsite-porphyry.  The  number  of  these  embedded 
crystals  varies  within  the  widest  bounds,  so  that  there  is  every  tran- 
sition between  felsite  and  felsite-porphyry.  The  colors  vary  from 
white  to  gray,  red,  purple  and  brown.  When  the  color  is  very 
dark  gray,  dark  green,  or  black  the  rock  is  basalt,  the  common  effu- 
sive of  the  ferromagnesian  magmas  and  granular  rocks,  less  often 
seen  in  dikes,  sheets,  etc.  The  effusive  occurrences  of  felsites  and 
basalts  have  been  already  treated  under  volcanoes  and  eruptions. 
The  enormous  tracts  of  land  in  western  America,  in  India,  etc., 
flooded  by  outflows  of  basalt  have  been  mentioned. 

The  feldspathic  lavas,  here  called  felsites,  are  subdivided  by  petrographers 
into  groups  based  on  the  nature  of  the  feldspars  and  other  minerals,  as  shown 
by  microscopical  study  and  chemical  analysis.  They  are  as  follows: 


Chief  component  minerals 

Rock  name 

Equivalent      coarse- 
grained rock  in  petro- 
graphic  classification 

Equivalent    coarse 
rock  in  the  classifi- 
cation of  this  book 

Alkalic  feldspars  and  quartz. 

Lime-soda       feldspars      and 
quartz 

Rhyolite 
\  Dacite 

Granite 
Quartz-diorite 

Granite 
Granite 

Alkalic     feldspars,     little    or 
no  quartz  

Soda-lime  feldspars,  little  or 
no  quartz  

Alkalic  feldspars  and  nephe- 
lite 

I  Trachyte 
>  Andesite 
}  Phonolite 

Syenite 

Diorite 

/  Nephelite 
\      Syenite 

/  Syenite, 
\     mostly 

f  Syenite  and 
\     Diorite 

Nephelite- 
Syenite 

These  terms,  rhyolite,  andesite,  etc.,  are  constantly  used  by  geologists  on  the 
basis  of  microscopical  study  and  experience,  but,  usually,  the  distinction  be- 
tween the  varieties  cannot  be  accurately  made  without  such  study,  and  for 
a  general  term  felsite  is  used  for  the  group. 

Glassy  Rocks.  —  Volcanic  glasses  occur  on  the  surface  of  outflows  of  lava, 
as  thin  crusts,  or  where  a  lava  flow  has  been  very  quickly  cooled,  and  they 
are  mostly  limited  to  felsite-magmas.  Bright,  clean,  hard  volcanic  glass  is 
called  obsidian,  and  pitchstone  when  the  luster  is  duller  and  more  pitchy. 
Pumice  is  a  frothy  condition  of  the  glass.  The  spongy,  scoriaceous  forms 


334  TEXT-BOOK   OF   GEOLOGY 

seen  on  lava  are  apt  to  be  more  or  less  glassy.  In  past  times  obsidian  was 
much  used  by  primitive  peoples  in  making  weapons,  implements,  etc.  The 
ancient  Mexicans  were  especially  skillful  in  fashioning  knives  and  razors  from 
it.  Natural  glasses,  like  the  obsidian  of  the  Yellowstone  Park,  are  apt  to 
contain  crystallized  minerals  in  rounded  forms  with  radiating  structures, 
known  as  spherulites,  or  lining  shell-like  cavities  known  as  lithophysae 
(stone  bubbles). 


CHAPTER   XIII 
METAMORPHISM  AND  METAMORPHIC  ROCKS 

Definition  of  Metamorphism.  —  Observation  teaches  us  that,  in 
addition  to  the  igneous  and  sedimentary  rocks  previously  de- 
scribed, there  is  a  third  class  which  cannot  be  directly  referred  to 
either,  and  these  have  been  termed  the  metamorphic.  Further 
study  of  them  shows  that  in  some  places  these  rocks  may  be  found 
to  pass  gradually  into  those  whose  fossils  and  stratification  prove 
them  undeniably  of  sedimentary  origin,  while  on  the  other  hand, 
in  other  places,  the  metamorphic  grade  into  rocks  whose  characters 
show,  no  less  conclusively,  that  they  are  of  igneous  origin.  From 
this  we  learn  that  the  metamorphic  rocks  are  closely  allied  to  both 
of  the  other  classes  and  are,  indeed,  formed  from  them  by  processes 
it  is  our  purpose  to  investigate  and  study.  The  word  metamorphic 
means  changed  in  form,  and  metamorphism  is  used  as  a  general 
term  for  all  those  changes  by  which  the  original  characters  of  rocks 
are  more  or  less  completely  altered,  in  that  their  component  kinds 
of  minerals  and  textures  are  transformed  into  other  minerals  or 
textures,  or  both.  The  change  may  be  so  great  that  the  metamor- 
phic product  bears  no  resemblance  to  the  rock  from  which  it  was 
derived,  but  appears  like  one  of  a  new  kind.  Where  sedimentary 
rocks  have  been  thus  thoroughly  metamorphosed  they  are  much 
harder,  denser,  more  crystalline,  and  the  fossils  and,  perhaps,  even 
the  marks  of  stratification,  have  been  more  or  less  completely  oblit- 
erated. As  to  the  igneous  rocks,  the  particular  features  which  dis- 
tinguish them  may  disappear,  and  they  may  assume  a  banded 
appearance  and  cleavage  which  resemble  those  of  sedimentary 
kinds. 

Thus  limestone  may  pass  into  highly  crystalline  marble  with  consequent 
loss  of  color  and  disappearance  of  fossils;  basalt  may  be  converted  into  green, 
slaty  rocks  which  give  no  hint  of  their  original  igneous  nature.  All  stages  of 
transition  may  be  found  between  such  extremes,  but  under  metamorphic  rocks 
we  understand  that  the  changes  of  the  original  rocks  have  been  so  profound 
that,  as  stated  above,  their  original  characters  have  been  entirely  obliterated, 
or  nearly  so,  and  distinctly  new  kinds  of  rocks  formed.  The  various  changes 
which  rocks  undergo  from  the  effects  of  weathering,  are,  strictly  speaking,  to 

335 


336  TEXT-BOOK   OF  GEOLOGY 

be  classed  as  metamorphic.  But  they  have  been  already  considered  under 
the  work  of  the  atmosphere  and  the  production  of  soils,  therefore  these 
agencies,  and  the  soils  and  weathered  rocks  which  result  from  their  action,  are 
not  considered  in  this  place. 

Metamorphic  Agencies.  —  These  are  mechanical  movements  of 
the  earth's  crust,  downward  pressure  of  superincumbent  masses, 
horizontal  thrusts,  chemical  action  of  liquids  and  gases,  and  the 
effect  of  heat.  These  may  be  simplified  into  the  effects  of  move- 
ment, water  solutions,  and  heat,  and  to  produce  complete  meta- 
morphism  in  rocks,  probably  all  three  of  these  are  required,  though 
not  necessarily  all  to  the  same  extent.  For  sometimes  the  effect  of 
one  factor,  such  as  heat,  may  greatly  predominate,  that  of  liquids 
be  less  marked,  and  those  of  pressure  or  movement  be  quite  negli- 
gible. Thus  in  the  metamorphic  changes  induced  in  surrounding 
rocks  by  the  intrusion  of  a  body  of  molten  magma  in  the  form  of  a 
neck  or  stock,  heat  is,  perhaps,  the  main  agent,  while  pressure  is 
of  little  or  no  effect.  We  will  consider  the  different  agencies  in 
detail. 

Movement  and  Pressure.  —  Simple  downward  pressure,  some- 
times called  static  pressure,  as  exerted  in  the  upper  part  of  the 
earth's  crust  by  the  weight  of  the  overlying  layers,  appears  in 
many  places  to  have  had  little  metamorphic  effect  on  rocks.  It 
must  tend  to  consolidate  sediments  by  bringing  the  grains  together, 
but  instances  are  known  where  strata  have  been  buried  for  long 
geologic  periods  under  great  thicknesses  of  overlying  beds  without 
having  suffered  any  notable  metamorphic  change,  as  may  be  ob- 
served where  they  have  been  gently  raised  and  exposed  by  erosion. 
On  the  other  hand,  in  various  places,  level-lying  strata,  which  have 
not  been  folded  or  crushed,  and  have  not  been  apparently  subjected 
to  very  great  vertical  pressure,  now  appear  highly  metamorphosed. 
They  are  generally  of  great  geological  age.  In  this  case  we  may  sus- 
pect that  great  heat  has  aided  the  pressure  in  effecting  the  change.  It 
is  also  natural  to  suppose  that  the  enormous  pressure  which  reigns  at 
greater  depths  in  the  shell  must  be  a  powerful  factor  in  inducing  cer- 
tain chemical  work  and  effects  in  rock  formation,  but  we  have  at 
present  no  definite  information,  or  means  of  learning  about  them, 
and  our  ideas  on  the  subject  are  mostly  in  the  nature  of  speculation. 

With  respect  to  movement  it  is  otherwise.  The  crust  of  the 
earth,  as  we  shall  see  later,  has  been  in  many  places,  at  different 
times,  under  great  compressive  force  which  has  found  relief  by 
wrinkling  up  the  outer  shell  into  mountain  ranges.  By  this  moun- 
tain-forming force  whole  masses  of  strata,  often  with  included 


METAMORPHISM    AND   METAMORPHIC   ROCKS  337 

igneous  rocks,  intrusive  and  extrusive,  are  folded,  crushed,  and 
mashed  together  in  the  most  involved  and  intricate  manner.  Not 
only  are  they  subjected  to  vast  pressure,  but  also,  in  the  mashing, 
to  enormous  shearing  stresses,  which  produce  forced  differential 
movements  among  the  rock  particles.  The  shear  may  be  character- 
ized as  a  sliding  of  the  particles  upon  themselves,  so  that  a  slipping 
of  adjacent  layers,  relative  to  one  another,  is  produced,  while  mash- 
ing is  a  process  of  flowage  by  which  adjacent  layers  become  thinner, 
but  do  not  slide  one  upon  another.  It  is  particularly  this  mashing 
and  shearing  which  are  of  great  power  in  producing  metamorphism. 
The  visible  effects  may  often  be  seen  by  the  manner  in  which  large 
crystals,  included  pebbles,  or  fossils,  are  flattened  and  elongated,  in 
the  plane  at  right  angles  to  the  mashing  force.  It  is  possible,  in 
fact,  for  mashing  and  shearing  alone  to  produce  rocks  having  the 
characteristic  outward  metamorphic  texture,  without  change  in  the 
original  mineral  composition,  but  in  combination  with  heat  and 
water,  this  is  of  the  highest  importance  in  inducing  chemical 
changes,  and  the  production  of  new  minerals.  It  is,  indeed,  notice- 
able that  as  long  as  rocks  retain  their  original  position,  they  may 
be  unaltered,  but  as  we  commence  to  find  them  disturbed  by  com- 
pressive  mountain-making  forces,  they  begin  to  show  signs  of  meta- 
morphism, and  largely  in  proportion  to  the  degree  to  which  they 
have  been  folded  up,  mashed,  and  sheared,  they  become  more  and 
more  metamorphosed.  The  effects  produced  in  this  way  are  com- 
monly referred  to  as  dynamic  metamorphism,  or  dynamo-meta- 
morphism.  It  should  be  understood,  however,  that,  in  the  use  of 
this  term,  not  pressure  alone,  but  the  action  of  solutions  and  heat 
is  also  included. 

Heat.  —  The  effect  of  heat  as  a  metamorphic  agent  is  very 
strong,  as  is  well  seen  where  intrusive  igneous  rocks  have  come  in 
contact  with  sedimentary  ones  and  metamorphosed  them.  It  in- 
creases greatly  the  solvent  action  of  solutions ;  it  tends  to  break  up 
the  chemical  compounds  of  many  minerals,  and  to  thus  make  new 
combinations.  The  heat  needed  for  metamorphism  may  possibly  in 
part  be  that  of  the  interior  of  the  earth,  it  may  be  supplied  in  part 
by  the  transformation  of  energy  resulting  from  the  movements,  the 
folding  and  crushing  of  the  rock  masses,  but  it  is  probably  mostly 
due  to  the  intrusion  of  molten  magma,  which  is  apt  to  rise  and  be 
intruded  into  rock  masses  when  they  are  uplifted  and  folded. 

Liquids  and  Gases.  —  The  most  important  of  these  is  water, 
which,  with  heat  and  pressure,  becomes  a  powerful  chemical  agent. 
It  acts  as  a  solvent,  and  helps  crystallization,  and  in  the  making  of 


338 


TEXT-BOOK   OF   GEOLOGY 


new  chemical  compounds.  It  is  aided  in  its  action  by  material  it 
may  carry  in  solution,  such  as  alkalies,  and  by  volatile  substances 
coming  from  the  magma  intrusions,  such  as  various  acid-forming 
substances,  fluorine,  for  example,  as  already  explained  under  vol- 
canic action.  This  explains  the  presence  in  metamorphic  rocks  of 
minerals,  which  contain  chemically  combined  the  elements  of  water 
and  other  gases,  and,  as  we  shall  see  later,  the  veins  and  ore  deposits 
frequently  found  in  them. 

Effect  of  Depth.  —  The  outer  shell  of  the  earth  may  be  divided 
into  different  zones,  according  to  the  various  geological  activities 


Soil 

Weathered  Rock 
Level  of  Ground  Water 


Zone  of  Fracture 
Belt  of  Cement 
Filled  with  Water 


»        I 
ation  V 

ter      } 


(6) 


ssures  Closed 
Bottom  of  Ground  Water 


Zone  of  Rock  Flowage 
Bottom  unknovoi 


Fig.  255.  —  Diagram  to  illustrate  different  zones  in  the  earth's  outer  shell. 

going  on  at  different  levels.  They  are  illustrated  in  the  diagram 
shown  in  Fig.  255.  Below  the  upper  layer  of  soil  the  bed-rock  is  full 
of  fractures,  and,  as  far  down  as  the  surface  of  ground  water,  it  is 
exposed  to  atmospheric  agencies,  the  moisture,  carbonic  acid,  etc., 
which  tend  to  cause  decay  and  convert  the  rocks  into  soil.  It  is 
a  zone  of  rock  destruction,  as  shown  in  the  first  chapter,  and  may 
be  called  the  belt  of  weathering,  (a) ,  Fig.  255. 

Below  this  belt  comes  the  zone  (b)  in  which  the  rocks  are  full  of 
cavities  and  fractures  filled  with  water.  The  upper  limit  is  that 
of  the  surface  of  ground  water,  the  lower  that  where  openings 
cease,  as  described  in  the  next  zone.  It  may  be  called  the  zone  of 
fracture,  if  we  disregard  the  upper  limit.  In  this  the  action  of 
water  is  most  important,  it  performs  chemical  work,  aided  by  the 
carbonic  and  other  acids  it  may  carry.  The  tendency  is  for  the 
silicate  minerals  of  the  rocks  to  change  into  those  containing  water 
in  combination,  or  into  carbonates.  Substances  are  taken  into 
solution,  and,  added  to  by  those  leached  downward  from  the  belt 


METAMORPHISM   AND   METAMORPHIC  ROCKS  339 

of  weathering,  are  deposited  in  the  fissures  and  pores  of  the  rocks. 
From  this  cementing  and  filling  of  cavities  the  zone  may  be  termed 
the  belt  of  cementation. 

Below  this  comes  the  zone  (c) ,  Fig.  255,  where  the  pressure  of  the 
superincumbent  masses  is  greater  than  the  elastic  limit  of  the 
strength  of  rocks;  they  crush  under  it,  and  are  to  be  regarded  as 
being  in  a  relatively  plastic  condition.  As  a  result,  all  openings 
and  fractures  are  closed,  and  this  must  mark  the  limit  downward 
of  the  percolation  of  ground  water.  The.  upper  limit  of  this  zone 
is  variable  and  depends  on  geological  conditions;  in  times  of  quiet 
it  may  be  15  miles  below  the  surface ;  in  times  of  compression  and 
mountain-making  it  may  be  at  a  much  lesser  depth.  Where  its 
lower  level  may  be  we  do  not  know.  In  this  zone  the  enormous 
pressure  and  increasing  heat  of  the  earth  are  the  chief  agencies; 
liquids  and  gases  are  less  important  and  tend  to  be  squeezed  out; 
thus  carbonates  change  to  silicates  and  C02  is  expelled.  The  new 
mineral  compounds  must  tend  to  be  of  smaller  volume  and  higher 
specific  gravity  from  condensation.  This  zone  of  rock  flowage  we 
may  term  the  zone  of  constructive  metamorphism. 

It  is  chiefly  in  the  lower  part  of  the  belt  of  cementation,  or  zone 
of  fracture,  and  in  the  upper  part  of  the  zone  of  rock  flowage  that 
the  work  of  producing  metamorphic  rocks,  as  we  know  them,  is 
done. 

The  zone  of  rock  fracture  has  been  called  the  zone  of  katamorphism,  and 
that  of  rock  flowage  the  zone  of  anamorphism,  by  Van  Hise;  the  meaning 
of  these  terms  is  that  one  is  a  downward  change,  or  breaking  up  into  simpler 
compounds,  while  the  latter  is  an  upward  change  or  reconstruction  into  more 
complex  compounds,  or  minerals.  These  terms  are  frequently  used  by  geol- 
ogists in  referring  to  metamorphism  and  metamorphic  rocks. 

Regional  and  Local  Metamorphism.  —  It  should  be  stated  be- 
fore proceeding  further  that  for  practical  purposes  two  varieties  or 
effects  of  metamorphism  are  recognized  by  geologists.  In  the  first 
the  rocks  are  changed  or  metamorphosed  over  extensive  regions; 
all  the  different  factors  of  metamorphism  have  worked  upon  them; 
but  pressure,  with  mashing  and  shearing,  has  been  especially  impor- 
tant, and  as  a  result  the  rocks  are  apt  to  have  a  peculiar  texture 
which  distinguishes  them,  and  which  will  be  presently  described.  In 
recognition  of  the  wide  extent  to  which  rocks  may  thus  be  changed 
this  has  been  called  regional  metamorphism,  and  sometimes  general 
metamorphism,  and  the  dynamic  metamorphism  referred  to  pre- 
viously is  merely  a  pronounced  phase  of  it  in  one  direction. 

But,  on  the  other  hand,  rocks,  especially  sedimentary  ones,  may 


340  TEXT-BOOK   OF  GEOLOGY 

be  metamorphosed  by  the  heat,  liquids,  and  gases  issuing  from  in- 
trusive molten  magmas  which  may  come  in  contact  with  them. 
Here  it  is  evident  that  mashing  and  shearing  are  unimportant; 
the  rocks  are  wanting  in  the  characteristic  texture  mentioned  above 
and  described  beyond,  but  may  yet  be  thoroughly  changed;  thus 
chalk  may  be  altered  to  marble.  With  reference  to  the  fact  that 
metamorphism  produced  in  this  manner  is  confined  to  the  immediate 
neighborhood  of  the  igneous  rocks  which  have  produced  it  and, 
therefore,  compared  with  regional  metamorphism,  is  limited  in  ex- 
tent, it  is  known  as  local,  or  very  commonly  contact  metamorphism. 
We  will  go  on  with  the  consideration  of  regional  metamorphism,  and 
local,  or  contact  metamorphism  will  be  treated  at  the  end  of  this 
chapter. 

Minerals  of  Metamorphic  Rocks.  —  The  chemical  compounds,  which  form 
the  minerals  found  in  the  rocks,  vary  greatly  in  their  ability  to  withstand  the 
changes  of  conditions  which  different  geological  processes  subject  them  to. 
With  new  chemical  and  physical  factors  operating  upon  them,  they  will  tend 
to  change  into  new  minerals,  or  those  chemical  combinations  which  will  be 
the  most  stable  under  the  new  conditions.  Thus  we  see  feldspar  alter  into 
clay  and  other  substances  through  the  action  of  water  and  carbon  dioxide,  as 
explained  under  soils,  page  25.  The  igneous  rocks  are  characterized  by  one 
set  of  minerals,  chiefly  silicates,  while  carbonates  and  hydrated  oxides  and 
silicates  are  mostly  found  in  the  sedimentary  rocks.  Some  minerals  like 
quartz  have  a  wide  range  of  stability  and  are  found  in  all  three  classes  of 
rocks,  but  many  minerals  when  subjected  to  metamorphic  processes  are  con- 
verted into  other  minerals;  thus  carbonates  are  apt  to  be  changed  into  sili- 
cates. Quartz,  the  feldspars,  mica  and  hornblende  are  found  in  both  igneous 
and  metamorphic  rocks,  while  common  garnet,  staurolite,  cyanite,  talc,  chlorite, 
and  serpentine  are  common  minerals  of  metamorphic  varieties. 

Texture.  —  Most  metamorphic  rocks  resemble  the  igneous  in 
that  they  are  highly  crystalline,  but  they  differ  from  them  in 
possessing  a  parallel  structure  which  may  at  times  closely  resemble 
stratification.  This  parallel  structure  expresses  itself  to  a  variable 
extent  by  a  foliated,  laminated,  or  as  it  is  often  termed,  a  schistose 
texture,  and  a  rock  possessing  it  is  known  as  a  schist.  By  reason  of 
it  a  rock  tends  to  split,  or  cleave,  more  or  less  perfectly  in  the  direc- 
tion of  a  plane  passing  through  it,  which  we  may  call  the  plane  of 
cleavage.  Although  it  is  the  characteristic  texture  of  metamorphic 
rocks,  which  for  this  reason  have  been  sometimes  called  the  crystal- 
line schists,  there  are  a  few,  such  as  serpentine,  marble  and  quartz- 
ite,  which  may  not  show  any  trace  of  this  parallel  schistose  structure 
and  cleavage.  In  some  cases  the  parallel  structure  is  straight,  or 
nearly  so,  as  seen  in  Fig.  256,  for  considerable  distances ;  often,  how- 


METAMORPHISM  AND   METAMORPHIC  ROCKS  341 


Fig.  256.  —  Banded  gneiss,  Portland   Township,  Ottawa   Co.,  Quebec. 
M.  E.  Wilson;    Geol.  Surv.  of  Canada. 


.  OF. 

ever,  the  banding  is  very  much  contorted,  bent,  or  curled,  showing 
the  kind  of  mashing  and  kneading  the  original  rocks  were  sub- 
jected to.  See  Fig.  259. 

Observation  shows  that  the  schistose  texture  is  due  to  the  arrangement  of 
unlike  mineral  grains  in  layers  or  flattened  lenses,  or  to  parallel  grouping  of 
prismatic  or  tabular  minerals,  such  as  hornblende  or  mica,  or  to  a  combina- 
tion of  both.  It  is  a  result  of  the  granulation  and  recrystallization  to  which 
the  original  rocks  were  subjected,  and  has  been  imposed  upon  igneous  and 
sedimentary  rocks  alike.  The  resemblance  of  the  banded,  laminated  appear- 
ance of  schistose  metamorphic  rocks  to  stratification  led  in  the  past  to  the 
erroneous  view  that  they  were  wholly  derived  from  stratified  ones;  that  they 
could  also  be  made  from  igneous  rocks  was  learned  much  later. 

Cleavage.  —  The  cleavage  which  is  exhibited  by  schistose  meta- 
morphic rocks  is  most  perfectly  developed  in  slates,  so  much  so  that 
this  variety  of  it  is  often  spoken  of  as  slaty  cleavage.  Slates  used 
for  roofing,  blackboards,  and  other  purposes  are  examples  of  this. 
Its  origin  has  been  the  cause  of  much  speculation  and  the  subject 
of  investigation,  along  both  experimental  and  mathematical,  as  well 
as  geological,  lines.  From  this  it  has  become  clear  that  it  is  the 
result  of  great  pressure  upon  the  material,  and  that  the  planes  of 
cleavage  are  at  right  angles  to  the  direction  of  pressure.  When  fine- 
grained sediments,  muds  and  clays,  are  subjected  to  intense  pressure, 
oblong  particles  tend  to  rotate  so  that  their  lengths  are  perpendicu- 
lar to  the  direction  of  pressure;  they  also  tend  to  become  flattened 
perpendicularly  to  it.  More  important  is  the  fact  that  many  of 
the  elongate  or  flattened  minerals,  such  as  mica,  kaolin,  chlorite,  and 
hornblende,  have  an  excellent  cleavage  parallel  to  the  long  or  flat 
directions.  All  these  features  tend  to  give  the  rock  a  capacity  to 
cleave  readily  in  one  direction.  Slaty  cleavage  is  thus  partly  molec- 
ular, when  it  passes  through  a  single  mineral  particle,  and  partly 
mechanical,  when  it  passes  between  arranged  or  unlike  particles. 
A  considerable  part  of  the  minerals  may  not  be  original,  such  as  the 
micas,  but  formed  by  the  metamorphism  accompanying  the  pres- 
sure. 

The  cleavage  planes  do  not  necessarily  bear  any  definite  relation 
to  those  of  original  bedding.  The  beds  were  laid  down  in  horizon- 
tal position  and  the  direction  of  pressure  is  also  horizontal  or 
nearly  so;  the  cleavage  planes,  being  at  right  angles  to  this, 
may  cut  the  bedding  at  right,  or  highly  inclined  angles.  But  as  the 
beds  may  be  folded  before  the  pressure  becomes  intense,  the  cleav- 
ages may  pass  through  the  bedding  at  various  angles,  though  they 
themselves  are  strictly  parallel,  see  Fig.  257,  Fig.  258,  and  also 
Fig.  260. 


METAMORPHISJVl  ,  AND  METAM&RPHIC   ROCKS 


343 


Although  by  far  the  greater,  mimber  6i  "slates  Jiave  been  made  from  original 
fine  sediments,  muds  and  clays-,  Islaty  rocks  *ha<y>ej,als(>  been  produced  by  the 
mashing  of  fine-grained -igneous  rocks,  such  as  f elites  and  basalts,  and  beds  of 


Fig.  257.  —  Slaty  cleavage  in  folded  beds. 

volcanic  ash.  In  this  process  original  characteristic  features  in  the  rocks  may 
become  greatly  distorted  and  even  obliterated;  thus  fossils  and  pebbles  in  the 
stratified  rocks  and  embedded  crystals  and  other  structures  of  the  igneous  may 
be  flattened  into  lenses  or  squeezed  out  into  cylinders.  And  it  must  also  be 


Fig.  258.  —  Slaty  cleavage  cutting  at  a  high  angle  beds  folded  in  a  syncline. 
ington,  Pa.     E.  B.  Hardin,  U.  S.  Geol.  Surv. 


Slat- 


remembered  that  what  has  been  here  said  of  cleavage  in  slates  is  equally  true 
of  other  schistose  rocks  in  which  it  differs  only  in  degree  of  becoming  less 
perfectly  developed. 

At  times  cleavage  may  be  mistaken  for  original  bedding,  unless  care  is  taken, 
and  wrong  interpretations  of  geological  structure  obtained.  It  is  sometimes 
important  to  indicate  it  on  geologic  maps  and  this  may  be  done  by  observing 
its  dip  and  strike,  like  that  of  a  bedding  plane.  The  important  relation  it  bears 
to  mountain  ranges  and  their  origin  will  be  discussed  under  that  subject. 


344  TEXT-BOOK   OF  GEOLOGY 

Places  of  Occurrence.  —  Metamorphic  rocks  are  widely  dis- 
tributed over  the  earth's  surface,  and  in  some  regions  they  are  the 
only  kinds  exposed  over  extensive  areas.  This  is  true  in  eastern 
Canada,  where,  in  places,  considerable  bodies  of  intrusive  igneous 
rocks  are  associated  with  them.  They  are  found  quite  generally  in 
New  England,  in  the  Adirondacks  and  in  a  strip  of  country  running 
from  northern  New  Jersey  to  Georgia.  Other  occurrences  will  be 
mentioned  in  the  second  part  of  this  work.  There  is  reason  also  for 
thinking  that  over  the  continental  areas  they  must  form  the  base- 
ment upon  which  all  the  later  unmetamorphosed  stratified  rocks 
rest.  For  wherever  these  latter  are  sufficiently  cut  away  by  ero- 
sion this  metamorphic  basement  appears,  except  where  it  has  been 
cut  into  by  intrusion  of  stocks  and  bathyliths  of  granite  and  other 
igneous  rocks.  The  metamorphic  rocks  also  form  the  interior  core 
of  many  mountain  ranges,  and  have  been  exposed  by  erosion.  These 
mountain  ranges,  as  will  be  discussed  later,  have  been  made  by  fold- 
ing of  the  strata,  and  in  proportion  to  the  intricacy  of  the  folding 
and  mashing,  so  is  the  degree  of  metamorphism  of  the  rocks  in- 
creased. 

The  relation  between  folding,  elevation,  and  metamorphism  is  so  well  estab- 
lished that,  where  we  find  rocks  intricately  folded  and  very  metamorphic,  we 
assume  that  an  elevation  once  existed  but  has  been  eroded  away,  or,  in  general, 
that  metamorphic  rocks  can  only  be  exposed  at  the  surface  after  continued 
erosion.  Following  out  this  idea,  metamorphic  rocks  are  sometimes  spoken  of 
as  continental  rocks,  because  they  imply  (when  originally  of  stratified  kinds) 
continued  erosion  of  land  masses;  laying  down  of  beds  of  sediments,  and 
folding  and  crushing  of  the  latter  to  give  metamorphism,  with  incidental 
production  of  mountain  ranges;  and  lastly  erosion  again  to  expose  the 
metamorphic  rocks.  Such  an  array  of  processes  could  occur  only  on  a  great 
scale  and  therefore  on  and  about  continental  masses;  consequently  when 
metamorphic  rocks  are  found  in  place  on  Fiji,  New  Caledonia,  South  Georgia 
and  other  islands,  it  is  held  that  this  proves  that  these  are  really  exposed  por- 
tions of  fragmented  continental  masses.  See  page  117. 

Age  of  Metamorphic  Rocks.  —  Some  of  these  facts,  previously 
mentioned,  led  to  the  view  that,  from  the  geological  standpoint, 
metamorphic  rocks  must  be  very  old.  This  by  no  means  necessarily 
follows,  nor  is  it  always  true.  For  while  we  find  almost  unmodified 
beds  of  quite  early  geologic  age  in  Russia  and  in  the  upper  Missis- 
sippi valley,  which  have  been  changed  but  little  from  their  original 
horizontal  position,  on  the  other  hand,  strata  of  a  comparatively 
recent  period  which  have  been  greatly  folded  up  in  the  Alps,  the 
Coast  Range  and  in  some  other  mountains  have  been  strongly  meta- 
morphosed. It  merely  depends  on  whether  they  have  been  subjected 


METAMORPHISM   AND   METAMORPHIC   ROCKS 


345 


to  metamorphic  processes  or  not,  and  the  older  the  rocks  are,  the  more 
likely  they  are  to  have  suffered  from  them.  Time,  however,  is  one 
of  the  most  important  factors  in  metamorphism,  and,  even  where 
relatively  recent  strata  have  been  changed  and  then  exposed,  the 
time  involved,  from  our  standpoint,  is  immensely  long. 


Kinds  of  Metamorphic  Rocks 

Introductory.  —  Since  the  metamorphic  rocks  are  made  from 
both  the  stratified  and  the  igneous,  and  there  are  almost  infinite  va- 
rieties of  both  of  these,  it  follows  that  there  must  be  a  very  great 
number  of  different  kinds  of  metamorphic  products.  Yet  in  the 
same  way  that  we  were  able  for  ordinary  purposes  to  gather  the 
igneous  and  stratified  rocks  into  a  small  number  of  groups,  so  we 
can  consider  the  metamorphic  under  the  few  most  important  types. 

The  sedimentary  strata  are  very  largely  made  of  disintegrated 
igneous  rocks.  In  the  process  of  breaking  down  and  disintegration 
it  may  happen  that  there  is  not  much  weathering  and  chemical 
change.  In  this  case  the  sedimentary  deposit  will  not  differ  very 
greatly  from  the  original  igneous  rock  in  chemical  and  mineral  com- 
position. Thus  the  red-brown  sandstone  (arkose)  of  the  Con- 
necticut valley,  which  is  full  of  feldspar,  has  practically  the  same 
composition  as  the  masses  of  granite  of  the  adjacent  region.  If 
such  arkoses  are  so  thoroughly  metamorphosed  as  to  lose  their 
original  characters,  it  is  evident  that  we  could  not  distinguish  them 
from  the  metamorphosed  granites,  or  determine  what  their  former 
status  was.  From  this  it  will  be  clear  that,  while  in  some  cases  we 
can  tell  the  origin  of  a  metamorphic  rock  at  once,  as  in  marble  and 
quartzite,  and  in  others  after  careful  study  in  the  field  and  labora- 
tory, in  many  cases  we  are  unable  to  ascertain  from  what  they  were 
derived. 

Classification.  —  Remembering  the  simple  classification  of  the 
sedimentary  rocks  previously  given,  it  is  possible,  in  a  general  way, 
to  show  the  relation  between  the  most  common  and  their  meta- 
morphic derivatives  in  the  following  table : 


Sediments 

Compacted  strata 

Metamorphic  rocks 

Gravel  

Conglomerate.  .  . 

Gneiss,  and  various  schists 

Sand  

Sandstone  

Quartzite,  and  various  schists 

Silt  and  clay  

Shale  

Slate,  and  various  schists 

Lime  deposits 

Limestone 

Marble,  and  various  schists 

346  TEXT-BOOK   OF   GEOLOGY 

In  the  case  of  the  igneous  rocks,  recalling  that  they  may  be 
roughly  divided  into  two  main  groups,  the  one  chiefly  composed  of 
light-colored  feldspathic  minerals,  and  the  other  mostly  of  dark 
ferro-magnesian  ones,  we  can  illustrate  also,  in  a  very  rough  and 
general  way,  the  relation  between  them  and  their  metamorphic  de- 
rivatives in  the  following  table : 


Igneous  rocks 

Metamorphic  rocks 

Coarse-grained    feldspathic    types,     such    as 

Gneiss 

Fine-grained  feldspathic   types,    such   as  fel- 
site  tuffs  etc 

Slate  and  Schists 

Ferromagnesian  rocks,   such  as  dolerites  and 
basalt                            

}  Hornblende-schists,        various 
J  schists,  and  serpentine 

Comparison  of  the  tables  will  show  that  gneisses  and  schists  may 
have  diverse  origins,  as  previously  pointed  out.  Combining  the  re- 
sults of  these  tables,  we  may  obtain  the  following  main  groups  of 
metamorphic  rocks,  distinguished  according  to  their  mineral  com- 
position or  by  their  texture,  or  by  a  combination  of  both. 

Grouping  of  Metamorphic  Rocks 

1.  Gneisses,  rocks  containing  feldspar. 

2.  Mica-schist  and  quartzite. 

3.  Slates  and  phyllites. 

4.  Hornblende- schist;  talc-,  and  chlorite-schists. 

5.  Marble,  dolomite,  mixed  carbonate-silicate  rocks. 

6.  Serpentine.    Iron-ores. 

Gneiss.  —  The  common  varieties  of  this  rock  (pronounced  nice) 
consist,  like  granite,  of  quartz,  feldspar,  and  mica,  but  in  gneiss 
the  mica  is  arranged  in  more  or  less  definite  planes  and  the  rock  has 
thus  a  rude  cleavage.  Sometimes  hornblende  may  accompany  or 
replace  the  mica,  and  other  minerals,  such  as  garnet,  may  also  occur, 
giving  different  varieties.  They  are  variable  in  color  from  light  to 
dark,  and  are  fine  to  coarse  in  grain.  All  degrees  of  transition  be- 
tween granite  and  gneiss  are  very  common,  and  in  other  cases,  where 
gneisses  were  made  from  conglomerates,  the  original  pebbles  may 
still  show  as  lenticular  masses.  Gneiss  is  one  of  the  most  common 
of  metamorphic  rocks,  and  it  appears  also  as,  perhaps,  the  most 
deeply  situated  of  any  known  rock,  formed,  probably,  in  the  deepest 


METAMORPHISM   AND   METAMORPHIC   ROCKS  347 

zones  of  metamorphism.  Some  rocks  are  spoken  of  as  granite- 
gneisses,  and  it  seems  probable  that  in  some  cases  the  gneissoid  tex- 
ture has  been  assumed,  through  mashing  and  shearing,  by  magmas 
while  still  in  a  pasty,  viscous  condition.  A  view  of  beds  of  gneiss 
is  seen  in  Fig.  259. 


"71 


Fig.  259.  —  Contorted  gneiss;   Fullerton,  Hudson  Bay,  Canada.     A.  P.  Low.     Geol. 

Surv.  of  Canada. 

Mica-schists  and  Quartzite.  —  When  a  sandstone  becomes  so 
firmly  cemented  by  deposit  of  silica  that  the  fracture  takes  place 
through  the  grains  of  quartz  sand,  instead  of  around  them,  it  has 
become  a  quartzite.  In  this  condition  it  shows  no  schistose  struc- 
ture, but  is  massive.  If  subjected  to  mashing,  a  quartz  schist  may 
be  formed;  or,  if  it  is  impure  sandstone,  mica  is  liable  to  develop 
and  may  increase  in  amount  until  it  appears  the  most  prominent 
ingredient,  and  the  rock  becomes  mica-schist.  All  degrees  of  tran- 
sition between  gneisses,  quartzites,  and  mica-schists  may  be  found, 
depending  on  the  relative  quantities  of  quartz,  feldspar,  and  mica. 
Quartzites  are  usually  massive  rocks  of  light  colors,  white,  gray, 
reddish,  or  buff,  and  of  hard  flinty  aspect.  Mica-schist  is  a  very 
schistose,  often  friable  rock,  usually  of  a  silvery  luster,  and  often 
dotted  with  common  red  garnets. 

Slates.  —  The  origin  of  these  rocks  from  fine-grained  sediments 
such  as  muds,  clays  and  ash  deposits  by  the  action  of  compressive 
forces  has  been  already  discussed.  While  they  may  have  various 


348 


TEXT-BOOK   OF   GEOLOGY 


colors,  red,  green,  gray,  etc.,  the  most  common  one  is  dark  gray 
to  black,  due  to  carbonaceous  material  from  organic  matter  in  the 
original  muds.  As  is  well  known,  they  are  quarried  for  roofing 
slates,  blackboards,  and  other  purposes.  See  Fig.  260.  They  are 
closely  related  to  shales,  but  the  distinction  between  them  is  that 
such  cleavage  or  lamination  as  a  shale  may  possess  is  due  to  original 
bedding  planes;  whereas  in  slates  it  is  a  secondary  induced  pheno- 
menon, which,  as  previously  stated,  may  bear  no  relation  to  bed- 
ding. Slate  is  sometimes  called  argillite  in  reference  to  its  origin 
from  clay  (Greek  argillos, clay). 


Fig.  260.  —  Illustrates  the  occurrence  of  slates  and  cleavage.     Slate  quarries.     Browns- 
ville, Me.     T.  N.  Dale,  U.  S.  Geol.  Surv. 

Phyllites.  —  These  are  rocks  resembling  slates,  but  having  a  larger  propor- 
tion of  mica,  which  gives  them  a  silky,  glimmering  luster.  They  are  transi- 
tional between  slate  and  mica-schist.  The  name,  which  means  "leaf  stone," 
has  been  given  for  the  remarkable  cleavage,  or  fissile  character,  of  these  rocks. 
They  appear  in  some  cases  to  have  been  formed,  like  the  ordinary  slates, 
from  sediments,  but  to  be  more  highly  metamorphic  and  recrystallized.  In 
other  instances  they  have  been  made  from  igneous  material,  felsite  lavas,  tuffs, 
etc.,  by  mashing,  shearing  and  accompanying  agencies  of  metamorpm'sm. 
They  have  sometimes  been  called  "hydro-mica-schist"  in  America. 

Schists :  Hornblende-schist.  —  There  is  a  great  variety  of 
rocks,  depending  on  different  minerals  which  compose  them,  which 
have  a  more  or  less  pronounced  foliated,  or  schistose,  structure. 


METAMORPHISM    AND    METAMORPHIC   ROCKS  349 

Schist,  as  previously  mentioned,  is  a  general  name  for  the  group, 
and  hornblende-schist  may  be  taken  as  a  most  common  and  typical 
representative.  The  rock  is  generally  dark  green  to  black,  and  the 
parallel  prisms  of  hornblende,  if  not  too  large,  usually  give  it  a 
silky  luster.  The  rock-cleavage  is  almost  slaty  in  some  cases.  Talc- 
schist  and  chlorite-schist  are  other  common  varieties,  in  which  talc 
and  chlorite  are  predominant  minerals. 

Marble.  —  This  is  the  metamorphic  condition  of  the  sedimentary 
rocks  formed  by  lime  deposits,  such  as  limestone  and  chalk.  Gen- 
erally, the  marks  of  bedding,  fossils,  etc.,  are  effaced  and  the  ma- 
terial converted  into  crystalline  grains  of  calcite.  It  is,  therefore, 
harder,  more  compact,  with  purer  colors,  and  takes  a  good  polish. 
Just  as  there  are  ordinary  limestones  consisting  only  of  carbon- 
ate of  lime,  and  dolomitic  limestones  containing  magnesium  car- 
bonate, MgC03,  in  variable  quantity  in  addition  to  the  CaC03,  so 
we  have  lime  marbles  and  dolomite  marbles.  Commercially,  as 
marble  is  used,  this  chemical  difference  is  not  a  matter  of  impor- 
tance, but  geologically,  it  is  of  interest  because  the  kinds  of  minerals 
that  are  liable  to  be  associated  with  the  marbles,  or  to  be  found, 
in  some  cases,  scattered  more  or  less  thickly  through  them,  are 
quite  different  in  the  two  kinds.  Marble  is  generally  massive  and 
shows  no  cleavage,  even  when  found  in  regions  where  its  association 
with  schists  shows  it  must  have  been  subjected  to  enormous  mashing 
and  shearing  stresses.  The  reason  for  this  appears  to  be  that  the 
mineral  calcite  has  a  curious  property  of  being  able  to  permit  of 
motion  of  its  molecules  in  certain  directions  without  the  crystals 
being  destroyed.  It  is  analogous  to  what  was  described  of  ice, 
page  132.  Owing  to  this  the  stresses  are  absorbed  molecularly,  and 
no  arrangements  of  the  grains  are  produced  which  show  as  foliation 
in  the  outward  structure,  as  they  do  in  schists. 

Pure  marble  is  white,  the  mottling,  banding  and  colors  shown  by  ornamental 
varieties  being  due  to  impurities,  the  red  and  yellow  tones  to  oxides  of  iron, 
the  grays  and  blacks  to  varying  proportions  of  organic  matter.  Besides  be- 
ing produced  by  regional,  marble  is  also  formed  by  contact  metamorphism. 

Serpentine.  —  This  name  is- given  to  a  mineral,  a  hydrous  silicate  of  mag- 
nesia, H4Mg3Si9Og,  and  also  to  a  rock  largely  or  entirely  composed  of  it.  The 
rock  is  usually  greenish  to  black,  soft,  of  a  greasy  feel,  and  massive,  or  without 
cleavage.  Some  of  the  blotched,  lighter  green  varieties  are  used  as  building 
and  ornamental  stones.  Most  serpentines  appear  to  have  been  made  by 
hydrothermal  metamorphism  (action  of  hot  waters)  on  deeply  buried  masses 
of  igneous  rock  rich  in  magnesia,  such  as  peridotite  for  example,  whereby  the 
magnesium  silicates  change  to  this  hydrated  variety.  Impure  dolomite 
marbles  may  contain  magnesium  silicates,  olivine,  pyroxene,  etc.,  which  may 


350  TEXT-BOOK   OF   GEOLOGY 

alter  to  serpentine.  'Verde  antique'  appears  in  some  cases  to  be  a  mixture 
of  marble  and  serpentine  of  this  nature. 

Iron-Ore.  —  The  mode  in  which  beds  of  iron-ore  may  be  accumulated  in  the 
stratified  rocks  has  been  already  described.  Such  iron-ores  may  be  subjected 
to  metamorphic  processes  like  other  rocks  and  as  a  result  the  loose  earthy 
materials  may  be  changed  to  hard  crystalline  rocks ;  thus  beds  of  limonite  and 
clay-ironstone  may  be  altered  to  hematite  and  magnetite. 

Anthracite,  or  hard  coal,  is  regarded  by  many  as  the  metamorphic  equiva- 
lent of  bituminous,  or  soft  coal.  The  degree  of  metamorphism  is,  however, 
very  slight;  were  coal  changed  in  proportion  as  the  other  metamorphic  rocks 
we  have  described,  it  would  be  converted,  not  into  anthracite,  but  into 
graphite,  which  is  not  combustible  in  the  air  under  ordinary  conditions. 


Local,  or  Contact,  Metamorphism 

Introductory.  —  As  previously  explained,  this  term  is  used  to 
denote  the  changes  which  are  induced  in  already  existent  rocks  by 
the  intrusion  into  them  of  a  mass  of  molten  magma,  and  also  the 
effect  of  the  contact  on  the  igneous  rock  which  the  magma  itself 
forms  in  cooling.  We  may  thus  observe  it  from  two  standpoints; 
that  of  the  result  on  the  igneous  rock-body,  termed  the  endomorphic 
effect,  and  that  of  the  action  on  the  enclosing  rocks,  or  the  exomor- 
phic  one.  Unlike  their  importance  in  regional  metamorphism, 
mashing  and  crushing  are  generally  negligible  factors,  and  heat  and 
the  action  of  vapors  and  liquids  are  the  chief  agents  in  producing 
the  changes  observed.  Therefore,  in  the  changed  rock,  while  new 
minerals  may  be  formed,  and  it  may  have  a  harder,  denser,  more 
crystalline  texture  than  the  normal  rock  of  the  region,  it  very  rarely 
shows  the  schistose,  or  cleavable  structure  as  a  result.  The  new 
rocks  are  massive,  and  not  schists,  except  as  they  may  retain  this 
structure  from  their  previous  condition. 

Endomorphic  Effect.  —  In  the  igneous  rock-body  itself  two 
effects  may  be  noticed  as  one  approaches  the  contact.  The  first 
and  most  usual  is  a  change  in  the  texture  of  the  intruding  rock.  It 
grows  much  finer  in  grain  and  at  the  contact  wall  may  be  very 
dense ;  thus  a  granite  may  change  to  a  f elsite.  The  reason  for  this 
is  the  quicker  crystallization  and  solidification  induced  in  the 
magma  by  the  chill  of  the  cold  rock-wall  with  which  it  comes  in  con- 
tact, as  explained  under  igneous  rocks,  page  325.  Sometimes  the 
igneous  rock  is  not  only  denser,  but  changes  from  an  evenly  granular 
to  a  porphyritic  texture  at  the  contact;  which  is  also  indicative  of 
more  rapid  cooling.  Sometimes  no  change  of  grain  is  visible,  and, 
in  this  case,  we  must  assume  that  the  rock-wall  was  thoroughly 
heated  by  the  flow  of  magma  past  it,  as  in  a  volcanic  conduit  for 


METAMORPHISM   AND   METAMORPHIC   ROCKS 


351 


example,  before  the  final  charge  of  magma  came  to  rest  against  it. 
In  such  circumstances,  the  magma  would  not  be  quickly  chilled, 
and  no  special  change  in  grain  might  be  expected.  But  in  this 
case  the  exomorphic  effects  are  usually  much  more  marked. 

The  other  effect  is  that  sometimes  new  minerals,  other  than  the 
normal  ones  of  the  igneous  rock,  may  be  found  occurring  at,  or  near, 
the  contact.  Thus,  for  example,  tourmaline  may  be  seen  not  infre- 
quently in  masses  of  granite.  The  origin  of  such  minerals  is  due  to 
the  chemical  effect  of  the  vapors  and  gases  of  the  magma,  which 
tend  to  be  excluded  as  the  mass  cools  and  crystallizes,  and  to  escape 
to  the  margin,  and  into  the  surrounding  rocks. 


Fig.  261.  —  Sections  of  intruded  stocks  and  their  contact  zones.  In  1,  the  breadth 
on  the  surface  CD  is  greater  than  AB,  depending  on  shape  of  igneous  mass.  In 
2,  the  width  F  is  greater  than  in  E,  depending  on  inclination  of  beds. 

Exomorphic  Effects.  —  The  most  noticeable  evidence  of  the 
exterior  effect  of  contact  metamorphism  is  a  baking,  hardening,  or 
toughening  of  the  surrounding  rocks  in  a  zone  surrounding  the 
igneous  mass.  As  a  result,  it  not  infrequently  happens  that  the 
altered  rocks  resist  erosion  better  than  the  intruded  igneous  body, 
or  the  unchanged  country  rocks,  and  form  projecting  topographic 
forms,  such  as  ridges,  peaks,  etc.  In  the  case  of  dikes,  it  may 
happen  that  both  the  dikes,  and  the  sedimentary  beds  penetrated 
by  them,  are  lowered  more  rapidly  by  erosion  than  the  hardened 
contact  rock  on  either  side,  leaving  it  standing  up  in  parallel  walls. 
In  most  cases,  however,  the  resistance  to  erosion  is  very  similar  to 
that  of  the  igneous  rocks. 

The  breadth  of  the  zone  depends  largely  on  the  size  of  the  igneous 
intrusion;  the  widest  and  most  pronounced  being  found  about  the 
great  stocks  and  bathyliths.  Around  them  it  has  been  observed  in 
many  places  that  the  contact  zone  may  reach  a  breadth  of  a  mile, 
or  even  more;  usually  it  is  some  hundreds  of  yards  and,  with  a 
small  intrusion  such  as  a  dike,  it  may  be  only  a  few  feet.  With  ex- 
trusive lava  flows  a  small  amount  of  baking  of  the  soils  or  rocks 
on  which  they  rest  is  often  noticed. 

Relation  to  Rock  Structure.  —  Around  an  intrusion  it  is  fre- 
quently observed  that  the  width  of  the  contact  zone  is  variable; 


352 


TEXT-BOOK   OF   GEOLOGY 


this  may  depend  very  much  on  the  position  of  the  rocks.  Thus  in 
Fig.  261,  section  1,  the  sloping  inclination  of  the  contact  wall  pro- 
duces a  wide  zone  at  CD,  compared  with  that  of  the  vertical  wall 
at  AB.  And  in  section  2  the  beds  at  F,  sloping  into  the  igneous 
rock,  tend  to  have  their  bedding  planes  opened,  and  to  furnish  an 
easy  entrance  to  the  vapors  and  solutions  from  the  cooling  magma. 
Since  these  vapors  and  solutions  are  the  chief  agents  in  carrying  the 
heat  and  producing  the  metamorphism,  it  is  clear  that  a  broad  zone 
F  will  be  made  on  this  side,  compared  with  E,  where  reverse  condi- 
tions are  present  and  a  narrower  zone  must  be  formed. 

Results  on  the  Different  Kinds  of  Rocks.  —  The  extent  to  which 
local,  or  contact,  metamorphism  produces  its  effects  upon  already 
existent  rocks  depends  very  much,  in  addition  to  what  has  been 
said  above,  upon  the  kinds  of  rocks.  It  should  not  be  forgotten 
in  this  connection  that  intrusions  of  igneous  rock,  such  as  dikes, 
may  take  place  into  older  igneous  rocks,  as  well  as  into  sedimentary 
strata.  The  latter,  as  a  rule,  are  much  more  profoundly  affected 
than  the  igneous  rocks.  For  our  purpose  here  the  sediments  may 
be  divided  into  the  three  groups,  the  sandstones,  the  shales  (and 
clays) ,  and  the  lime  rocks.  On  pure  sandstones  the  effect  is  rather 
small,  though  near  the  contact  they  may  be  changed  into  quartzite. 
The  lime  rocks  are  changed  into  marble,  of  greater  or  less  purity, 
the  masses  of  which  may  extend  for  considerable  distances.  In  the 
case  of  clays  and  shale  beds  the  most  notable  and,  generally,  far- 
reaching  results  are  seen,  the  soft  shales  being  greatly  hardened  and, 
finally,  at  the  contact,  converted  into  dense  crystalline  rock  known 
as  hornjels,  which  in  its  outward  appearance  may  strongly  resemble 
an  igneous  rock,  such  as  basalt.  The  igneous  rocks,  being  the  prod- 
ucts of  fusion,  are  generally  but  little  affected  by  later  intrusions, 
especially  feldspathic  kinds  like  granite. 

In  approaching  a  contact  zone  in  shales,  after  a  slight  hardening,  one  of  the 
most  noticeable  effects  is  the  production  of  spots,  or  knots,  in  the  rock. 
These  may  consist  of  small  points,  or  lumps,  or  the  production  of  prisms 
of  some  mineral,  such  as  andalusite  (Al2SiO5),  which  may  be  black  from 
included  carbonaceous  matter.  Still  nearer  to  the  contact,  and  at  it,  the 
knots  disappear  and  the  rock  has  a  granular  crystalline  appearance,  which  re- 
calls that  seen  in  the  igneous  rocks. 

The  most  interesting  results  are  produced  in  limestones,  especially  impure, 
cherty  varieties.  Not  only  are  they  turned  into  marble,  but  a  great  variety 
of  new  minerals  may  be  formed  in  them,  depending  on  reactions  between  the 
bases  and  acidic  oxides  present,  especially  lime  and  silica.  Thus  the  silica 
tends  to  drive  out  carbon  dioxide, 

CaC03  -f  Si02  =  CaSiO,  +  CO, 


METAMORPHISM    AND   METAMORPHIC   ROCKS  353 

and  calcite  is  changed  into  lime  silicate  (wollastonite) .  If  the  limestone  is 
dolomite,  then  magnesia  takes  part, 

(CaMg)C03  -f  Si02  =  (CaMg)Si03  +  CO2 

and  calcite  is  changed  into  pyroxene,  and  carbon  dioxide  liberated.  Clay  may 
be  present,  furnishing  alumina,  and  iron  oxides  may  also  occur;  while,  in  addi- 
tion to  the  water  vapor,  sulphur,  fluorine,  boron  and  other  acid-forming  ele- 
ments from  the  magma,  may  take  part,  and  thus  by  various  combinations, 
numbers  of  new  chemical  compounds,  or  minerals,  are  formed.  These  more 
complex  reactions  may  be  illustrated  by  the  following  example: 

Calcite      +  Clay  +  Quartz  =  Garnet  +  Carb.  dioxide  +  Water. 

3  CaC03  +  H4Al2Si2O9    +  SiO2       =  Ca3Al2Si3Oi2  +  3CO2  +  2H2O. 

Thus  a  limestone,  impure  with  clay  and  sand,  may  be  changed  into  garnet  with 
evolution  of  carbon  dioxide  and  water. 

It  seems  quite  certain  that,  in  addition  to  the  water  and  other  volatile  sub- 
stances, silica  is  carried  in  solution  into  the  enclosing  rocks;  taking  part  in  the 
chemical  reactions  mentioned  above,  cementing  them  by  deposit  in  their  pores, 
as  when  sandstone  is  converted  into  quartz  ite,  and  forming  veins  of  quartz  in 
crevices  and  fissures.  The  alkalies,  soda  and  potash,  are  also  carried  by  these 
solutions,  or  emanations,  from  the  magmas  into  the  surrounding  rocks,  and  by 
many  geologists  it  is  claimed  that  alumina,  iron,  magnesia  and  many 
other  elements  are  thus  transferred  in  contact  metamorphism.  It  seems  cer- 
tain that  in  many  cases  this  is  true,  but  as  yet  the  necessary  chemical  knowl- 
edge, which  would  permit  us  to  understand  exactly  what  takes  place,  has  not 
been  obtained.  The  importance  of  these  assumptions  we  shall  see  when  ore 
deposits  are  studied. 


CHAPTER  XIV 
THE  FRACTURES  AND  FAULTING  OF  ROCKS 

Fractures;  Joints 

General  Remarks.  —  The  fact  that  in  the  outer  shell  of  the 
earth  the  rocks  are  traversed  in  all  directions  by  fractures,  varying 
from  minute  crevices  to  important  fissures,  has  been  already  alluded 
to  in  many  parts  of  this  book.  We  have  seen  their  importance  in 
the  weathering  of  rocks  and  formation  of  soil ;  in  the  holding  and  in 
the  circulation  of  underground  water;  in  earthquakes,  and  in  some 
metamorphic  processes,  and  we  shall  meet  them  again  in  consider- 
ing mineral  veins.  They  are,  indeed,  of  great  geologic  importance, 
both  on  account  of  the  processes  which  give  rise  to  them  and  from 
the  results  which  are  achieved  by  their  aid.  It  is  fitting,  then,  that 
we  should  study  them  in  some  detail. 

Fractures  are  found  in  all  classes  of  rocks,  and  for  purposes  of 
study  they  may  be  divided  into  joints  and  rifts;  the  difference  be- 
tween them  is  one  of  degree,  the  joints  being  developed  in  a  single 
rock-mass  or  a  certain  set  of  strata,  while  the  greater  rifts  may 
traverse  many  adjoining  rock  masses  and  extend  to  great  distances. 
The  joints  show  little  or  no  dislocation  of  their  contiguous  walls; 
with  the  rifts  there  may  or  may  not  have  been  displacement,  but  at 
the  surface  their  existence  is  generally  revealed  to  us  by  differential 
movements  on  the  opposite  sides,  as  explained  later  under  faults. 
We  will  consider  the  joints  first. 

Our  knowledge  of  the  fractures  in  rocks,  gained  by  a  study  of  surface 
conditions,  has  been  greatly  extended  below  by  mining  operations.  Mining 
geologists  commonly  divide  the  fractures  into  the  lesser  or  joints,  as  defined 
above,  and  fissures,  for  the  greater.  The  latter  may  extend  considerable 
distances,  even  a  number  of  miles.  The  word  rift,  used  above  in  a  general 
way,  includes  such  fissures,  and  also  the  much  greater  fractures  which  divide 
the  outer  shell  into  vast  blocks,  and  are  illustrated  by  the  great  San  Andreas 
Rift  of  California,  mentioned  on  page  245,  or  in  the  great  Rift  Valley  of 
Africa,  see  page  241.  It  would  be  perhaps  well  to  use  rifts  for  such  vast 
fractures,  and  fissures  for  the  more  common  lesser  ones  of  the  mining 
geologists. 

As  ordinarily  used,  the  words  fracture,  fissure,  etc.,  imply  that  to  some 
extent  the  surfaces  of  rupture  are  not  in  absolute  contact,  that  some  opening 

354 


THE  FRACTURES  AND  FAULTING  OF  ROCKS 


355 


exists  between  them.  As  employed  by  geologists  this  may,  or  may  not,  be 
implied.  Joints  are  generally  tightly  closed;  they  may  gape  at  the  surface 
an  inch  or  more;  below,  such  separations  are  generally  filled  with  mineral 
matter.  Rifts  and  fissures  usually  show  some  separation,  in  most  cases  filled 
or  healed  by  deposited  material;  when  this  is  wanting  and  the  walls  are  in 
contact,  it  is  known  as  a  "tight  fissure"  or  fracture.  Fissures,  whether 
separated  and  filled,  or  tight,  may  exist  without  displacement,  as  seen  in  the 
mines  at  Cobalt,  Ontario,  or  they  may  exhibit  the  faulting  described  beyond. 


Fig.  262.  —  View  illustrating  joints  in  limestone  beds.     Drummond  Island,  Mich. 
I.  C.  Russell,  U.  S.  Geol.  Surv. 

Joints  in  Stratified  Rocks.  —  The  smaller  fractures  which  divide 
the  rock  masses  are  those  which  we  may  call  joints.  Examination 
generally  shows  that  they  are  present  in  systems ;  that  is,  they  run 
as  divisional  planes  through  the  strata  more  or  less  perfectly  parallel 
to  definite  directions.  Often  it  happens  that  these  directions  of 
jointing  are  two,  vertical,  or  nearly  so,  and  approximately  at  right 
angles,  and  this,  combined  with  the  natural  divisional  bedding 
planes,  divides  the  strata  into  series  of  closely  fitted  blocks.  Or  there 
may  be  three  or  more  systems  of  jointing.  The  finer  the  grain  of  the 
rock,  as  a  rule,  the  more  perfect  the  jointing.  Thus,  in  shale  beds 
and  in  limestones,  it  may  be  very  perfect,  as  illustrated  in  Fig.  262. 
Such  jointing  may  arise  through  various  agencies,  such  as  the  ten- 
sion produced  in  the  beds  of  sediments  by  the  contraction  which 
ensues  when  they  are  elevated  from  the  sea-bottom  to  form  land 
surfaces,  and  undergo  a  drying-out  process.  Or,  at  such  times,  or 
later,  the  beds  may  be  subjected  to  folding,  warping  and  torsional 


356  TEXT-BOOK   OF  GEOLOGY 

effects  through  crustal  movements,  by  which  the  joints  are  made  by 
cracking  in  regular  systems.  It  has  also  been  suggested  that  the 
passage  of  earthquake  waves  through  the  rocks,  with  the  sudden 
alternate  compression  and  tension,  is  the  cause  of  much  of  the 
minor  jointing  observed.  The  exact  cause  of  most  of  the  joints 
is  not  known,  but  they  have  sometimes  been  classified  as  tensional 
or  compressional  joints,  according  to  the  supposed  nature  of  the 
force  producing  them. 

Joints  are  a  matter  of  great  importance  in  all  quarrying,  tunneling,  and 
mining  operations  where  rock-work  enters  as  an  important  factor,  since  the 
jointing  obviously  greatly  facilitates  progress.  Otherwise,  every  rock  frag- 
ment would  have  to  be  broken  or  blasted  loose  from  bed-rock. 

In  regions  where  the  stratified  rocks  have  been  definitely  folded,  the  joints 
are  sometimes  the  result  of  tension  in  the  anticlines,  and  sometimes  of  com- 
pression in  the  folding.  Slaty  cleavage  is  thus  commonly  found  to  be  asso- 
ciated with  joints.  In  folded  strata,  when  parallel  with  the  strike  of  the 
beds,  or  nearly  so,  they  are  called  strike- j oints ;  when  at  right  angles  to  this, 
or  nearly  so,  they  are  called  dip-joints,  being  in  line  with  the  dip. 

It  is  often  noticed  that  joints  of  a  certain  system  in  disturbed  strata,  which 
are  probably  to  be  associated  with  the  folding,  extend  for  long  distances, 
through  a  whole  series  of  beds,  and  are  known  as  master- joints.  They  are 
contrasted  with  the  minor  joints  which  may  be  limited  to  a  single  stratum. 

Joints  in  Igneous  Rocks.  —  The  jointing  observed  in  igneous 
rocks  is  mostly  due  to  the  contraction  resulting  from  the  cooling  of 
the  heated  mass.  It  occurs  just  after  the  solidification  from  the 
molten  state,  when  the  loss  of  heat  from  the  newly  formed  solid 
is  greatest.  It  may  manifest  itself  in  one  of  several  ways,  depend- 
ing on  the  rate  of  cooling,  the  size  and  shape  of  the  igneous  body, 
and  other  things.  Thus  intrusive  masses  of  granite  and  other  rocks 
are  cut  by  jointing  planes  in  various  directions  which  divide  them 
into  large  blocks,  often  roughly  tabular,  or  into  prisms.  In  some 
cases,  especially  in  the  finer-grained  felsites  and  porphyries,  the 
jointing  in  sheets,  laccoliths  and  dikes  is  on  a  very  small  scale,  well 
shown  in  the  talus  coming  from  exposures  of  the  igneous  rock,  which 
consists  of  small  angular  fragments.  Sometimes  in  laccoliths,  and 
similar  dome-shaped  intrusions,  there  is  a  shelly  jointing  on  a  large 
scale,  parallel  to  the  domed  surface.  This  appears  to  have  been 
caused  by  the  planes  of  cooling  (and  parting)  having  descended 
evenly  into  the  mass  from  the  domed  surface. 

A  much  rarer  kind  of  jointing  seen  in  igneous  rocks  is  one  in  which  the  con- 
traction took  place  very  regularly  around  certain  centers,  producing  spherical 
masses  by  the  cracking.  This  kind  of  structure  is  especially  brought  out  by 
the  weathering  of  the  rock  mass.  It  occurs  both  in  intrusive  bodies  and  in 


THE  FRACTURES  AND  FAULTING   OF  ROCKS  357 

lava  flows.  Igneous  rock  masses  may  also  exhibit  jointing,  due,  as  in  other 
kinds  of  rocks,  to  tensional  and  compressive  stresses  in  the  earth's  crust. 
But,  as  they  are  previously  jointed  by  contractional  cooling,  as  explained 
above,  this  is  of  minor  importance,  since  the  stresses  are  more  likely  to  relieve 
themselves  by  movement  along  the  existent  joints  than  by  forming  new  ones. 

Columnar  Structure.  —  The  most  striking  method  of  jointing  in 
an  igneous  rock,  by  contraction  on  cooling,  is  shown  when  columnar 
structure  is  developed.  This  takes  place,  in  general,  when  the  ex- 
tension of  the  mass  is  great  in  two  directions  and  much  less  in  a 


Fig.  263.  —  "  Devil's  Post-pile";  columnar  jointing  in  lava.    Head  of  the  San  Joaquin 
River,   Cal.     H.  W.  Turner,  U.  S.  Geol.  Surv. 

third,  as  in  a  dike,  an  intrusive  sheet,  or  a  lava  flow.  The  rock- 
body  may  then  be  composed  of  a  series  of  closely  fitted  prisms, 
which  are  again  divided  by  cross  joints.  The  prisms  may  have  a 
variable  number  of  sides,  but  most  commonly  they  are  hexagonal, 
and  sometimes  of  wonderful  regularity  of  form.  They  may  be 
several  inches,  or  a  number  of  feet,  in  diameter,  and  from  one  foot 
to  200,  or  even  more,  in  length.  See  Fig.  263.  The  Giant's  Cause- 
way on  the  north  coast  of  Ireland  is  one  of  the  most  celebrated 
examples  of  this  columnar  structure.  The  columns  are  perpendic- 
ular to  the  chief  cooling  surface,  and  thus  in  a  level  intruded  sheet, 
or  in  a  flow  of  lava,  they  stand  vertically,  while  in  a  dike  they  tend 
to  be  horizontal,  that  is,  perpendicular  to  the  plane  of  greatest 
extension  of  the  rock  mass.  Thus  some  dikes,  exposed  as 
walls  by  erosion,  resemble  regularly  piled  cordwood.  In  other 


358  TEXT-BOOK  OF  GEOLOGY 

masses  their  position  depends  on  the  directions  taken  by  the 
cooling  planes;  in  volcanic  necks  they  may  be  perpendicular,  or  in 
them,  as  well  as  in  other  rock-bodies,  they  may  be  curved,  or  even 
radiant. 

The  cause  for  the  structure  appears  to  be  this.  When  the  igneous  mass  is 
cooling  slowly  and  regularly,  centers  of  cracking  tend  to  occur  on  the  cooling 
surface  at  equally  spaced  intervals.  From  each  interspace  three  cracks  will 
form  and  radiate  outward  at  angles  of  120°.  These,  intersecting,  produce 
regular  hexagons,  and  the  cracks  penetrating  inward  make  the  columns.  But, 
as  the  contractional  centers  are  not  always  equally  spaced,  four-/  five-,  and 
even  seven-sided  columns  occur.  The  columns  again,  contracting  lengthwise, 
break  into  sections.  The  same  principle  is  seen  in  the  manner  in  which 
drying  mud-flats  crack  into  polygonal  shapes.  See  Fig.  210. 

Jointing  in  Metamorphic  Rocks.  —  The  jointing  seen  in  these 
rocks  depends  largely  on  their  nature.  In  the  massive  gneisses  it  is 
very  much  like  that  in  granite,  while  in  the  very  fissile  and  schistose 
rocks,  such  as  slates  for  example,  it  is  more  like  that  observed  in 
many  sedimentary  beds.  In  general  it  may  be  said  to  resemble 
that  in  the  sedimentaries,  but  to  be  less  perfect;  as  a  rule  the  meta- 
morphic  rocks  are  apt  to  be  much  jointed. 

Great  Rifts.  —  In  addition  to  the  divisional  planes  in  the  rocks, 
which  have  been  described  above  as  joints,  they  are  penetrated  by 
fractures  or  rifts  on  a  great,  and  in  some  cases  vast,  scale.  The 
most  direct  evidence  of  these  great  rifts  is  seen  in  the  phenomena  of 
faults,  as  described  later,  but  the  indirect  evidence  of  their  exist- 
ence is  also  shown  in  a  number  of  ways.  /Thus,  the  alignment  of 
volcanoes  in  many  places  suggests  it  tpage  £17) ,  as  do  also  springs 
(page  157) ;  the  arrangements  of  drainage  in  some  places,  and  the 
direction  of  mountain  ranges  in  others,  also  lead  us  to  infer  their 
existence.  The  outer  crust  of  the  earth  appears,  indeed,  to  be 
everywhere  divided  into  great  blocks  by  these  fractures  or  rifts, 
see  pages  241  and  245.  Usually  the  walls  of  the  rifts  are  pressed 
tightly  together,  in  many  cases  they  are  healed  by  deposits  of 
mineral  matter  in  them.  In  a  few  instances  they  would  be  open 
save  for  the  debris  which  has  tumbled  into  them,  or  been  broken 
from  their  walls,  and  which  fills  them  up.  Any  further  discussion 
of  them  leads  us  inevitably  to  the  subject  of  faults. 

Faulting 

Faults.  —  When  rifts  have  been  formed  in  the  rock  masses  of 
the  outer  shell  of  the  earth,  movements  along  the  face  of  such  rifts 
may  occur  at  the  time  of  their  formation,  or  subsequently,  giving 


THE  FRACTURES  AND   FAULTING   OF   ROCKS  359 

rise  to  displacements  of  the  rock  masses,  compared  with  their  former 
positions.  Such  displacements  are  called  faults,  and  faults  are  a 
matter  of  great  importance  in  geology.  We  have  already  met  them 
in  discussing  movements  of  the  earth's  crust  and  earthquakes,  and 
we  shall  observe  them  playing  very  important  parts  in  our  con- 
sideration of  mountain  ranges  and  of  ore  deposits.  They  are  a  more 
or  less  constantly  recurring  feature  which  must  be  dealt  with  in 
the  proper  understanding  and  delineation  of  geologic  structures.  As 
geologic  phenomena  they  are,  therefore,  of  interest,  not  only  from 
the  scientific,  but  also  from  economic  and  technical  standpoints. 

Faults  are  frequently  described  and  treated  as  if  they  were  connected  only 
with  the  stratified  rocks.  This  is  a  mistake,  for  while  they  are  most  easily  ob- 
served in  such  rocks,  and  thus,  perhaps,  seem  to  occur  most  often  in  them, 
they  are  also  found  in  igneous  and  metamorphic  rocks  and  may  give  rise 
in  them  to  important  structures  and  be  of  great  technical  consequence. 

The  Fault-surface.  —  The  fracture  along  which  movement  and 
dislocation  has  occurred  is  often  spoken  of  as  the  fault-plane. 
While  it  is,  perhaps,  natural  to  speak  of  it  as  a  plane,  it  is  probably 
rarely  flat  for  any  distance,  but  more  or  less  warped,  broken,  and 
frequently  offset,  and  it  is,  therefore,  better,  and  causes  less  mis- 
apprehension, to  term  it  the  fault-surface.  Moreover,  the  move- 
ment in  faulting  may  occur,  not  upon  one  determined  surface,  but 
upon  a  number  of  more  or  less  closely  adjacent  ones,  producing  a 
fault  zone,  in  which  the  various  slipped  blocks  may  make  in  the 
aggregate  the  total  displacement.  Such  a  distribution  is  sometimes 
called  step- faulting.  The  masses  of  rock  involved  in  fault  move- 
ments are  generally  of  such  size  and  weight,  and  often  so  com- 
pressed together,  that  the  motion  of  one  fault  face  on  the  other,  along 
the  faulting  surface,  takes  place  under  tremendous  pressure.  As  a 
result  of  this  rubbing  under  pressure,  the  rock  faces  are  smoothed 
and  striated,  and  not  infrequently  beautifully  polished,  and  such 
polishings  and  groovings  are  known  as  slickensides.  The  line  of 
intersection  of  the  fault  with  the  plane  of  the  horizon  is  called  the 
strike,  or  trend,  of  the  fault,  just  as  we  speak  of  the  strike  of  the 
bedding  plane  of  upturned  sediments  (page  299).  The  surface  of 
faulting  is  rarely  exactly  vertical,  it  is  apt  to  be  inclined,  and,  in 
some  cases,  so  much  so,  that  it  may  approach  horizontality.  The 
angle  of  incidence  between  the  fault-surface  and  the  vertical  plane 
passed  through  the  line  of  strike  is  the  hade  (see  page  314) ;  the 
angle  with  the  plane  of  the  horizon  is  the  dip,  as  with  strata,  and 
this  is  the  complement  to  the  hade.  As  it  is  more  natural  to 


360  TEXT-BOOK  OF  GEOLOGY 

think  of  fissures  with  reference  to  vertical  directions,  hade  is  used, 
possibly,  more  conveniently,  though  perhaps  less  commonly,  than 
dip.  In  the  case  of  an  inclined  fault  the  side  which  tends  to  be 


Fig.  264.  —  Diagram  to  show  fault  terms. 

above  is  known  as  the  hanging  wall,  the  other  as  the  foot-wall. 
See  Fig.  264.  If  one  were  to  imagine  the  fissure  opened  and  himself 
descending  it,  the  appropriateness  of  these  old  mining  terms  becomes 
obvious. 


Fig.  265.  —  Fault  in  shale;  the  drag  and  curvature  of  the  beds  show  that  the  left  side 
has  gone  down,  the  right  up.     Little  River  Gap,  Tenn.     A.  Keith,  U.  S.  Geol.  Surv. 

A  fault  is  not  infrequently  composed  of  numerous  parallel  ruptures  and  slips. 
While  the  fracture  is  generally  tightly  closed  it  may,  on  occasion,  have  opened 
and  been  filled  with  fragments  from  above,  or  the  grinding  of  the  walls  upon 
one  another  may  produce  a  zone  of  broken  material,  and  such  angular, 
crushed  rock  filling  a  fault  is  known  as  fault-breccia.  Examination  often 
shows  also  that  when  stratified  beds  are  faulted,  as  shown  in  Fig.  265,  there  is  a 
curvature  of  them  at  the  fault-surface  resulting  from  the  drag.  Such  curva- 


THE  FRACTURES  AND  FAULTING  OF  ROCKS 


361 


ture,  as  illustrated,  may  be  a  useful  aid  in  determining  the  directions  of 
motion  on  the  opposite  sides. 

Motion  on  the  Fault-surface.  —  Experience  shows  that  if  we 
consider  one  side  of  a  fault  to  stand  fast,  motion  on  the  other  side 
may  be  up,  down,  side-wise,  or  obliquely.  Thus  in  Fig.  266  .the 
lettered  plane  may  represent  the  fault  face,  which  for  convenience 
we  may  consider  to  remain  at  rest,  the  other  side  which  undergoes 
motion  having  been  removed  to  expose  it.  If  we  suppose  some 


£-4- 


Fig.  266.  —  Diagram  to 
show  possible  motion  in 
faulting. 


Fig.    267.  —  Normal    and 
reverse  faults. 


particle  A,  for  instance  a  pebble,  to  be  cleft  by  the  fault-fracture, 
then  one  part  A  remains  in  its  original  place  and  the  other  part  B, 
embedded  in  the  other  face,  may  be  carried  in  some  direction  by  the 
faulting.  This  line  A-B  is  the  direction  and  amount  of  the  fault. 
B  may  be  carried  from  A  vertically  up  or  down  on  G-H,  or  hori- 
zontally on  C-D,  or  in  any  radial  direction,  and  usually  is  taken  in 
some  more  or  less  oblique  course  A-B. 

Normal  and  Reverse  Faults.  —  If  we  consider  faulting  as  having 
taken  place  merely  in  a  vertical  plane  then  two  important  cases  may 
arise.  In  A,  Fig.  267,  the  hanging  wall  has  apparently  slipped  down 
with  reference  to  the  foot- wall;  a  fault  of  this  kind  is  known  as  a 
normal  fault.  In  the  other  case,  B  in  the  figure,  the  hanging  wall 
has  apparently  been  crowded  up  over  the  foot- wall,  and  a  fault  of 
this  kind  is  called  a  reverse  fault.  It  will  be  noticed  that  with  the 
normal  fault  a  particular  layer  V-V  has  been  lengthened  apparently 
by  an  amount  corresponding  to  the  gap  C-E,  while  in  the  reverse 
fault  it  has  been  shortened  by  an  equivalent  overlap.  With  refer- 
ence to  what  has  been  supposed  to  be  their  origin,  normal  faults  are 
sometimes  called  tension  faults  and  reverse  ones  compression  faults. 


362 


TEXT-BOOK   OF   GEOLOGY 


It  will  be  observed  that  in  the  above  statement  normal  and  reverse  faults  are 
said  to  be  apparently  formed  by  vertical  up  or  down  movements.  There  can 
be  no  doubt  but  that  in  many  cases  the  direction  of  movement  is  approxi- 
mately vertical,  that  is,  along  G-H,  Fig.  266,  or  nearly  so,  but  in  very  many 
other  cases  it  is  not;  the  motion  is  oblique  along  some  line  A-B,  and  we  may 
even  have  normal  and  reverse  faults  formed  by  a  simple  horizontal  shove 
along  C-D,  Fig.  266.  This  may  be  seen  by  careful  observation  of  Fig.  268. 
The  movement  of  the  front  block  to  the  right  has  produced  an  apparent 


B    F 


Fig.  268.  —  To  illustrate  how  normal  faulting,  as  seen  in  a  vertical  plane  PP,  may  be 
caused  by  simple  horizontal  shoving  on  the  fault-surface  FF.  The  particular 
stratum  B  in  PP  (right-hand  figure)  appears  to  have  slipped  down.  Modified  from 
Ransome. 

normal  fault;  had  the  movement  been  to  the  left  we  should  have  had  an 
apparent  reverse  fault.  The  terms  normal  and  reverse  should,  therefore,  not 
be  used  in  the  sense  of  conveying  ideas  of  particular  kinds  of  motion,  but 
merely  to  indicate  the  results  achieved,  as  shown  in  a  vertical  cross  section 
of  the  faulted  parts. 

Components  of  Faulting.  —  In  order  that  we  may  be  able  to 
understand  and  define  the  geologic  structure,  where  faulting  has 
occurred,  it  is  necessary  that  we  should  know  the  amount  and  direc- 
tion of  what  we  may  term  the  components  of  a  fault.  This  may  be 
understood  by  aid  of  the  adjoining  diagram,  Fig.  269.  In  this 
A-G-E-F  represents  the  horizontal  plane,  A-F  is  the  strike  of  the 
fault,  that  is,  its  intersection  with  the  horizontal  plane,  and 
A-D-B-F  is  the  fault-surface.  Let  us  suppose  that  the  motion  has 
been  such  that  a  particle  A  has  been  carried  to  the  position  B,  then 
the  line  A-B  joining  these  two  positions  is  the  displacement,  or  slip; 
and  no  matter  what  path  the  particle  may  have  followed,  A-B  is  the 
resultant,  and  its  length  the  measure  of  the  slip.  The  line  A-B, 
however,  in  order  that  it  may  be  fixed  and  determined,  must  be  re- 
ferred to  known  axes  and  this  is  done  by  referring  it  to  three 
planes  at  right  angles  to  each  other.  The  first  is  the  plane  of  the 
horizon,  A-G-E-F,  the  second  is  a  vertical  plane  parallel  to  the 
strike,  E-G-D-B,  and  the  third  is  the  vertical  plane,  F-E-B-H,  at 


THE  FRACTURES  AND  FAULTING  OF  ROCKS 


363 


right  angles  to  the  last  one.  Now  the  line  F-E  gives  the  amount  of 
motion  along  the  horizontal  plane  at  right  angles  to  the  strike  F-A, 
and  this  is  known  as  the  heave  of  the  fault,  the  line  E-B  is  the 
amount  of  vertical  motion  and  is  called  the  throw  of  the  fault,  while 
F-A,  or  E-G,  the  amount  of  motion  along  the  strike  in  the  hori- 
zontal plane,  may  be  termed  the  shove,  or  strike-slip  of  the  fault. 


Fig.  269.  —  Diagram  to  illustrate  and  define  the  components  of  faulting.      Fault- 
surface  shaded. 

The  intersections  of  the  three  planes  give  the  three  right-angled 
axes  F-E}  E-B,  and  E-G,  meeting  in  the  common  point  E,  and 
these  may  be  termed  the  component  axes  of  faulting.  The  direc- 
tions and  intercepts  on  these  axes  being  known,  the  displacement 
can  be  calculated,  and  the  problem  of  the  fault  solved. 

The  heave  and  throw  of  faults  are  the  components  commonly  recognized, 
as  we  shall  presently  see  in  considering  them  in  stratified  rocks.  The  reason 
is  that  the  dislocation  is  most  easily  seen  in  a  vertical  section  A-G-D-C ,  and 
in  this  the  particle  A  has  apparently  moved  from  A  to  D,  while  the  amount  of 
shove  B-D,  or  A-F,  may  not  be  at  all  evident  on  the  surface.  The  shove  is, 
indeed,  as  a  rule,  difficult  to  estimate  in  most  faults  and  often  it  cannot  be 
determined  at  all. 

It  is  clear  that  a  fault  might  take  place  without  shove,  the  movement  being 
wholly  in  the  vertical  plane  A-G-D-C ;  it  might  also  take  place  with  a  verti- 
cal fault-surface  E-G-D-B  and  in  this  case  there  would  be  no  heave;  there 
might  be  shove,  but  this  might  also  be  wanting  and  it  would  be  a 
pure  throw  fault.  We  might  also  have  a  simple  shove,  without  throw  or 
heave.  The  last  case,  in  which  the  fault-plane  is  horizontal  and  there  is  pure 
heave  without  throw  or  shove,  while  theoretically  possible,  hardly  seems  prac- 
ticable, the  nearest  approach  to  it  are  certain  faults  described  later  under 
thrusts. 

A  good  case  of  the  throw  of  a  normal  fault  is  seen  in  Fig.  188,  and  of  shove 
without  throw  in  Fig.  189. 


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Faults  in  Stratified  Rocks.  —  Although  faults  occur  in  all  kinds 
and  combinations  of  rocks,  they  are  best,  and  therefore,  most  fre- 
quently, observed,  in  stratified  beds,  on  account  of  the  strongly 


ABC 

Fig.  270.  —  Model  illustrating  strike-faulting  in  stratified  rocks.     A,  before  faulting; 
B,  after  faulting,  fault-scarp  still  uneroded;  C,  surface  levelled  by  erosion. 

marked  stratification  which  they  disarrange.  With  relation  to  this 
structure,  faults  may  be  strike- faults,  when  the  strike  of  the  fault 
and  that  of  the  strata  are  parallel,  or  nearly  so,  as  illustrated  in 
Fig.  270;  or  they  may  be  dip-faults ,  when  at  right  angles  to  the 


ABC 

Fig.  271.  —  Model  illustrating  dip-faulting:    A,  before  faulting;    B,  after  faulting, 
fault-scarp  uneroded;   C,  surface  levelled  by  erosion,  showing  offsets  of  strata. 

strike  of  the  strata,  or  nearly  so,  as  shown  in  Fig.  271;  or  they 
may  be  oblique  faults,  when  at  45°  to  the  strike  of  the  beds,  or 
approximately  so. 


Fig.  272.  —  Diagram  illustrating  concealment  of  strata  by  strike-faulting. 

In  the  cases  shown  in  the  figures  the  faults  are  normal  ones ;  they 
may  also,  of  course,  be  reversed  faults.  They  are  also  depicted 
without  real  shove,  yet  it  will  be  noted  in  Fig.  271  C  that,  appar- 
ently, shove  has  occurred,  causing  the  beds  to  offset.  This  sudden 


THE  FRACTURES  AND  FAULTING  OF  ROCKS 


365 


offsetting  of  strata,  traced  along  their  strike,  is  one  of  the  surest 
indications  of  a  dip-fault.  Strike-faults  are  more  difficult  to  per- 
ceive and  may  be  easily  overlooked;  they  may  cause  deception  as 
to  the  thickness  of  strata  by  producing  apparent  repetitions.  See 
Fig.  270  C.  Thus,  in  traversing  strata  a  repetition  of  a  certain  set 
should  lead  to  suspicion  of  strike-faulting.  On  the  other  hand, 
strike-faults  may  conceal  strata  after  erosion  has  occurred.  Thus 
in  Fig.  272,  where  a  reverse  fault  FF  has  occurred  with  movement 
from  C  to  B  and  subsequent  erosion,  there  is  no  outcrop  of  the 
stratum  A  at  the  surface. 


Fig.  273.  —  Diagram  to  illustrate  rotary  motion  in  pivotal  faults.     A,  before  faulting; 
B,  after  faulting;    C,  effect  on  outcrop  after  erosion. 

Rotary  Faults.  —  The  movement  of  one  side  of  a  fault-face  on  the  other 
side  may  be  attended  by  rotary  motion,  as  illustrated  in  Fig.  273,  showing  the 
original  and  final  positions  of  the  fault-faces  as  projected  on  a  vertical  plane. 
Faults  of  this  nature  are  known  as  rotary  faults.  They  are  sometimes  indi- 
cated, when  strike-faults,  by  the  strike  of  the  strata  on  opposite  sides  of  the 
fault-line  not  being  parallel,  and  in  dip-faults  by  a  sudden  change  in  the 
direction  of  the  strike  of  strata  as  the  fault-line  is  crossed. 


Fig.  274.  —  Illustrating  the  forming  of  a  trough  by  normal  faults. 

The  Magnitude  of  Faulting.  —  The  scale  on  which  faulting  has 
taken  place  varies  within  the  widest  bounds.  It  may  be  but  a 
fraction  of  an  inch  as  illustrated  in  Fig.  204,  it  may  be  a  number  of 
feet  and  from  this  up  to  many  thousands.  Normal  faults  attain 
these  magnitudes  in  the  Appalachians,  while  in  the  Plateau  region 
faults  of  several  thousands  of  feet  throw  are  not  uncommon,  the 
fractures  extending  for  hundreds  of  miles,  and  the  fault-scarps,  being 
yet  uneroded,  form  lines  of  cliffs  which  give  character  to  the  topog- 


366 


TEXT-BOOK   OF  GEOLOGY 


raphy.  The  Great  Basin  region  presents  on  a  colossal  scale  the 
phenomenon  of  faulting,  the  area  between  the  Sierra  Nevada  on 
the  west  and  the  Wasatch  on  the  east  being  divided  into  huge 
blocks  by  fractures  running  for  many  miles,  and  the  sinking  of  these 
blocks  has  produced  faults  of  great  dimensions.  This  will  be 
further  alluded  to  under  mountains.  Sunken  tracts  of  country  due  to 
normal  down-faulting,  as  illustrated  in  Fig.  274,  form  what  are 
called  troughs  (German  graben,  French  fosse)  and  are  illustrated  in 
the  valley  of  the  Rhine,  the  great  Rift  Valley  of  Africa  with  its 
lakes,  and  many  other  places.  Such  troughs,  graben,  are  the  direct 
opposite  to  horsts,  which  are  mentioned  on  page  243.  The  relation 
between  them  and  igneous  outflows  and  intrusions  is  mentioned  on 
page  225. 


Fig.  275.  —  A  thrust-fault  on  a  small  scale.     Near  Houston,  Okla.     J.  A.  Taff, 

U.  S.  Geol.  Surv. 

Thrusts  and  Thrust-faulting.  —  Reverse  faults  are  most  com- 
monly found  in  those  regions  where  crushing  and  folding  of  the 
earth's  shell  has  taken  place,  and  the  stronger  the  folding  or  crush- 
ing has  been,  the  greater  and  more  evident  the  reverse  faults  are. 
Thus,  it  is  especially  in  the  stratified  rocks  in  mountain  regions  that 
these  results  are  seen,  as  in  the  southern  Appalachians.  The  careful 
and  detailed  study  of  old  mountain  areas  has  disclosed  the  fact  that 


THE  FRACTURES  AND   FAULTING  OF  ROCKS  367 

these  reverse  faults  have  sometimes  occurred  on  a  tremendous  scale, 
and  with  the  fault  having  a  comparatively  low  angle  of  inclination, 
even  being  in  some  cases  nearly  or  quite  horizontal.  Reverse  faults 
thus  having  a  gently  inclined  to  horizontal  fault-surface  are  known 
as  thrust-faults  or  simply  thrusts,  see  Fig.  275,  and  they  may  be 
of  such  magnitude  and  importance  that  by  some  geologists  they  are 
considered  quite  aside  from  faults,  and  in  a  class  by  themselves. 
The  fault-surface  in  this  case  is  spoken  of  as  a  thrust-plane. 

Such  thrusts  have  been  discovered  and  studied  especially  in  the  Alps,  in 
Scotland,  in  the  northern  part  of  the  Scandinavian  peninsula,  in  the  southern 
Appalachians  and  in  the  front  range  of  the  Rocky  Mountains  in  Montana 
and  British  Columbia.  The  distances  which  the  lower  formations  may  be 
pushed  and  made  to  over-ride  the  later  ones  are  sometimes  amazingly  great, 
ranging  from  a  number  of  miles  up  to  70,  or  100,  or  even  more.  In  Fig.  276 
is  seen  a  section  representing  a  portion  of  the  great  thrust  along  the  front 
ranges  of  the  Rocky  Mountains  in  northern  Montana.  The  deciphering  of 
such  great  displacements  is  one  of  the  triumphs  of  modern  geological  research. 

Lewis  Range 


Fig.  276.  —  Section  showing  the  thrust  in  northern  Montana,  whereby  very  old 
geologic  formations  of  the  Algonkian  are  made  to  over-ride  the  much  younger 
beds  of  the  Cretaceous.  BB  is  the  surface  of  thrusting;  DD  and  Chief  Mountain 
are  erosional  remnants  of  the  Algonkian  resting  on,  and  surrounded  by,  the  younger 
Cretaceous.  Displacement  by  thrusting  observed,  7  miles;  total  amount  unknown. 
Generalized  after  Willis. 

Topographic  Results  of  Faulting.  —  If  a  fault  of  some  consider- 
able magnitude  were  to  occur  suddenly,  it  would  naturally  be 
marked  at  the  surface  by  a  corresponding  displacement,  giving  rise, 
if  vertical  or  nearly  so,  to  a  cliff,  which  is  commonly  called  a  fault- 
scarp.  Such  fault-scarps  are  not  unknown,  and  have  been  described 
from  a  number  of  places.  The  connection  of  quickly  formed  faults 
with  earthquakes  has  been  previously  alluded  to  (page  244)  and  an 
illustration,  Fig.  277,  shows  a  fault-scarp  which  has  just  been 
formed  with  resultant  earthquake  shock. 

Such  scarps  may  be  called  initial  fault-scarps.  As  the  process  of 
weathering  and  erosion  works  more  actively,  in  general,  on  the  up- 
lifted side,  the  scarps  tend  to  become  dissected,  eroded,  lowered,  and 
to  retreat  from  the  fault-line.  Thus  they  may  pass  through  youth- 
ful, mature,  and  old  stages.  Finally,  the  difference  in  elevation  on 
opposite  sides  of  the  fault-line  may  be  worn  away  completely,  and 
thus  all  topographic  expression,  initially  due  to  faulting,  may  be 


368 


TEXT-BOOK   OF   GEOLOGY 


Fig.  277.  —  Waterfall  due  to  sudden  forming  of  a  fault-scarp  across  a  stream  bed, 
part  of  movement  which  caused  a  great  earthquake.  Balboa  Bay,  Alaska.  W.  W. 
Atwood,  U.  S.  Geol.  Surv. 

obliterated.     This  would  finish  one  cycle  of  erosion  on  a  faulted 
surface.    See  Fig.  278. 

If  now  the  whole  mass  should  be  uplifted  with  little,  or  without 
relative,  displacement  of  the  parts  and  thus  a  new  cycle  of  erosion 
inaugurated,  similar  to  that  explained  under  the  rejuvenation  of 
rivers  and  river-work,  then  the  agents  of  erosion  might  find  on 
opposite  sides  of  the  fault-line  rock  structures  of  quite  different 
hardness  and  ability  to  withstand  their  attack.  Thus,  one  side 
might  be  lowered  so  much  more  rapidly  than  the  other  as  to  leave 
the  latter  standing  as  a  cliff  or  escarpment.  This  latter  would  be 
due,  however,  not  to  the  initial  faulting  movement,  but  to  subse- 
quent differential  erosion  in  the  following  cycle  on  opposite  sides 
of  the  fault-surface.  Such  cliffs  deserve,  therefore,  a  different  name 


THE  FRACTURES  AND  FAULTING   OF   ROCKS  369 

and  have  been  termed  fault-line  scarps  by  Professor  Davis.  They 
may  develop  on  the  side  of  the  block  that  was  originally  uplifted, 
and  are  then  termed  by  him  resequent,  or  they  may  form  on  the 
opposite  block,  and  face  toward  the  uplifted  side,  and  are  then  called 
obsequent.  The  varying  resistance  to  erosion  on  the  opposite  sides  of 


ABC 

Fig.  278.  —  Shows  the  origin,  development,  and  history  of  an  initial  fault-scarp. 
A,  block  of  strata  containing  two  harder,  more  resistant  intruded  sheets  of  trap, 
before  displacement.  B,  after  faulting  and  some  erosion;  the  fault-scarp  has 
become  mature,  and  has  retreated  from  the  fault-line.  C,  approaching  the  end 
of  the  first  cycle  of  erosion;  the  fault-scarp  has  been  obliterated. 


ABC 

Fig.  279.  —  Development  of  fault-line  scarps.  A,  faulted  block  of  strata  commencing 
a  second  cycle  of  erosion;  intruded  trap  sheets  more  resistant  than  the  enclosing 
beds;  uplifted  block  to  the  right.  B,  after  erosion;  a  fault-line  scarp  has  formed 
which  faces  toward  the  uplifted  block  and  is  therefore  obsequent.  C,  continued 
erosion  has  carried  away  the  top  trap  of  B  and  its  obsequent  cliff  and  a  new  one 
has  formed  facing  the  other  way,  toward  the  sunken  block;  this  is  a  resequent 
fault-line  scarp;  compare  B,  Fig.  278. 

the  fault-line  determines  naturally  which  will  form,  and  the  course 
of  a  very  long  and  old  fault-line  might  be  marked  in  one  place  by 
resequent,  and  in  another  by  obsequent,  scarps.  Finally,  through 
the  completion  of  another  cycle  of  erosion  these  also  in  turn  might 
be  worn  away.  An  understanding  of  them  may  be  gained  by  ob- 
serving Fig.  279,  which  may  be  considered  a  later  development  of 
Fig.  278. 

The  east  slope  of  the  Sierra  Nevada,  the  west  slope  of  the  Wasatch,  and  the 
steep  faces  of  the  intervening  north  and  south  ranges  of  the  Great  Basin,  as 
previously  discussed,  are  held  to  represent  more  or  less  eroded  fault-scarps. 
At  the  west  base  of  the  Wasatch  Range  some  of  the  faulting  has  occurred  so 
recently  that  fault-scarps  may  be  seen  uneroded  in  the  soft  fans  of  alluvial 
niaterial  brought  down  by  the  streams.  The  Plateau  region,  through  which 


370  TEXT-BOOK   OF   GEOLOGY 

the  Colorado  River  cuts  its  way,  is  dominated  in  its  topography  by  a  series 
of  great  faults,  whose  almost  uneroded  scarps  form  prominent  cliffs.  They 
have  been  described  as  fault-scarps,  but  are  very  probably  fault-line  scarps 
developed  in  a  second  cycle  of  erosion.  One  of  the  most  striking  instances  of 
initial  fault-scarps  is  found  in  the  great  Rift  Valley  of  central  Africa  whose 
walls  form  prominent  escarpments  for  great  distances.  Examples  on  a 
smaller  scale  are  very  common.  Thus  the  sunken  tract  of  sandstones  and 
intercalated  trap  sheets  between  New  Haven,  Connecticut,  and  Springfield, 
Massachusetts,  is  divided  into  a  series  of  tilted  blocks  by  faulting.  It  has  passed 
through  one  cycle  of  erosion,  in  which  the  initial  fault-scarps  have  been 
eroded  away;  it  is  now  in  a  second  cycle  and  the  resistant  outcrops  of  trap 
form  prominent  ridges,  fault-line  scarps,  both  obsequent  and  resequent,  which 
dominate  the  topography  and  reveal  the  system  of  faulting  which  divides  the 
displaced  masses.  See  Fig.  281. 

Erosion  of  Faults.  —  On  the  other  hand,  it  is  also  true  that  in 
many  places  the  most  profound  faults  exist,  with  displacements 
amounting  to  thousands  of  feet,  of  which  there  is  no  trace  so  far  as 
the  surface  is  concerned,  both  sides  being  at  the  same  level.  We 
must  conclude  in  such  cases  that  great  erosion  has  occurred,  that 
the  first  cycle  has  been  completed,  or  possibly  that  the  growth  of 
the  displacement  has  been  slow  enough  to  be  controlled  by  it.  It 
seems  not  unreasonable  to  believe  that  the  latter  has  often  occurred, 
for  we  can  scarcely  imagine  that  the  formation  of  great  faults,  with 
thousands  of  feet  of  displacement,  has  been  a  sudden  process,  but 
rather  the  gradual  yielding  of  the  shell  of  the  earth  in  response  to 
the  forces  brought  to  bear  upon  it  during  long  periods  of  time.  The 
detection  of  eroded  faults,  which  may  be  a  matter  of  the  highest 
importance  in  understanding  the  structure  of  a  region,  is  often  one 
of  great  difficulty,  demanding  the  greatest  skill  and  geologic  knowl- 
edge. 

The  detection  of  faults  which  do  not  show  any  distinct  topographic  relief  is 
accomplished  in  a  variety  of  ways.  The  most  common  and  obvious  is  the  dis- 
turbance, or  discontinuity,  produced  in  the  structure  of  stratified  rocks,  as  pre- 
viously explained.  This  applies  to  metamorphic  rocks  also,  but  to  a  lesser 
extent,  because  their  structures  are  more  complicated  and  confused,  less  clear 
and  evident.  In  homogeneous  masses  of  igneous  rock  it  may  not  be  possible 
to  detect  faults,  yet  even  here  discontinuity  in  certain  features  which  they 
may  possess,  such  as  dikes  and  veins,  may  lead  to  the  discovery  of  faults  in 
them. 

Origin  of  Faults.  —  The  immediate  cause  of  faults  is  compara- 
tively simple  and  generally  agreed  upon;  they  are  due  either  to 
compression,  or  to  stretching,  of  the  outer  shell  of  the  earth.  In 
the  first  case,  through  the  stress  which  accumulates  from  the  in- 
creasing force  of  the  thrust,  the  rock  masses  are  strained  to  a  point 


THE  FRACTURES  AND  FAULTING   OF   ROCKS  371 

where  they  can  no  longer  resist,  but  must  give  way.  Relief  occurs 
through  readjustment  by  movement,  either  along  the  surfaces  of 
some  previous  fracture,  or  by  the  formation  of  a  new  one.  Some 
effects  produced  by  such  movements  have  been  considered  under 
earthquakes.  It  is  also  clear  that  faults  of  this  nature  will  occur 
chiefly  in  places  where  folding  of  the  strata  is  a  prominent  feature 
and  thus,  as  we  shall  see  later,  in  mountain  ranges.  If  the  proc- 
ess takes  place  on  a  great  scale,  we  may  have  overthrusts  developed. 
On  the  other  hand,  where  segments  of  the  outer  shell  are  (rela- 
tively) uplifted,  as  in  the  formation  of  horsts  (see  page  243),  the 
strata  may  be  under  tension,  and  the  stretching  find  relief  by  frac- 
turing and  faulting,  the  latter  produced  by  the  gravitative  settling 
and  readjustment  of  the  fault  blocks.  Thus  over  wide  regions  where 
the  strata  are  not  otherwise  disturbed,  as  in  the  Colorado  Pla- 
teau, they  may  be  penetrated  by  fractures  and  show  great  displace- 
ments along  them.  And  also  in  the  upper  portion  of  up-arching 
folds  there  may  be  tension  and  cracking,  with  subsequent  gravita- 
tive settlement  and  faulting. 

It  is  natural  to  think  that  in  regions  of  folded  rocks  reverse  faults  would 
be  the  chiet,  or  only  kind  developed,  but,  although  it  is  true  that  they  are 
essentially  confined  to  such  places,  the  converse  of  this,  that  normal  faults 
are  found  only  in  unfolded  strata,  is  by  no  means  the  case.  On  the  contrary, 
they  are  also  abundant  in  folded  and  dislocated  areas,  as  well  as  in  those 
where  the  strata  are  still  horizontal,  or  nearly  so.  The  great  majority  of 
faults,  in  fact,  appear  to  be  normal  ones,  due  to  gravitative  settling  for 
the  most  part,  and  resulting  in  elongation  of  the  crust,  and  to  thus  deserve 
the  name;  but  it  should  not  be  forgotten,  as  shown  on  a  previous  page,  that 
normal  and  reverse  are  only  terms  for  certain  results,  and  that,  for  example, 
an  apparently  normal  fault  may  be  produced  by  compression.  In  further 
explanation  of  what  has  been  said  above,  if  a  segment  of  the  earth's  shell 
subsides,  it  is  evident  that  the  beds  near  the  edges  of  the  block  will  be 
subjected  to  tension,  and,  eventually,  to  rupture  and  gravitative  faulting. 
The  same  is  as  true,  as  stated  above,  if  a  segment  rises,  the  relative  dis- 
placement and  stretching  being  the  important  feature.  Thus,  in  the  sinking 
areas  along  coast-lines  which  are  receiving  heavy  deposits  of  sediment,  such 
tensional  effects  must  occur.  Also  the  sinking,  or  rising,  of  such  areas  may 
be,  and  probably  is,  not  uniform  over  their  extent,  and  thus  torsional  stresses 
due  to  the  warping  will  be  set  up,  with  fracturing  and  readjustment  of  the 
blocks  and  consequent  faulting.  We  can  hardly  imagine  movements  of  the 
earth's  outer  shell  to  take  place  without  either  compression,  tension,  or  tor- 
sion occurring  and  producing  more  or  less  faulting.  As  a  consequence,  all 
parts  of  it  that  are  open  to  our  inspection  display  this  phenomenon  to  some 
extent. 

The  ultimate  cause  of  faulting  evidently  depends  on  those  proc- 
esses within  the  earth  which  give  rise  to  compression,  or  tension, 


372  TEXT-BOOK   OF   GEOLOGY 

and  to  movement  of  segments  of  its  outer  shell.  They  are  most 
strikingly  displayed  in  the  formation  of  its  chief  features  of  relief, 
in  mountain  ranges  for  example,  and  faulting  may  be  considered 
only  a  minor  and  attendant  result  of  their  operations.  We  shall 
wait,  therefore,  until  the  grander  results  of  these  processes  have  been 
discussed,  before  considering  them  and  venturing  from  the  known 
into  the  realm  of  the  unknown. 


CHAPTER    XV 
MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY 

Definition  of  Mountains.  —  No  exact  limit  can  be  set  as  to 
the  height  an  elevation  should  rise  above  the  surrounding  country 
in  order  that  it  may  be  properly  termed  a  mountain,  for  this  is 
largely  a  matter  of  comparative  relief,  and  the  mountains  of  one 
region  where  the  relief  is  small  would  be  only  hills  in  another  where 
it  is  great;  they  may  vary  from  a  few  hundred  feet  high,  up  to  the 
loftiest  summits  in  the  world.  Although  we  frequently  read  of  the 
"everlasting  hills,"  at  the  very  outset  it  should  be  understood  that 
mountains,  like  all  forms  of  terrestrial  relief,  are  not  permanent 
structures,  but  are  always  wasting  under  the  attack  of  atmospheric 
agencies,  though  not  infrequently  renewed  by  repetition  of  the  same 
processes  which  originally  caused  them,  as  will  be  shown  in  later 
discussions. 

The  Grouping  of  Mountains.  —  When  we  consider  the  arrange- 
ment of  mountains  we  find  that  they  may  be  irregularly  disposed  in 
groups,  such  as  the  Catskill  Mountains  in  New  York,  the  Judith 
Mountains  in  Montana,  the  Black  Hills  in  South  Dakota,  or,  as  is 
more  commonly  the  case,  they  may  be  aligned  in  ranges,  such  as  the 
Sierra  Nevada,  the  Caucasus,  etc.  Such  ranges  may  consist  of  a 
single  ridge,  but  more  often  they  are  compound,  composed  of  a 
variety  of  ridges  whose  general  direction  is  parallel,  giving  a 
united  trend  to  the  whole  range.  As  we  shall  see  later,  a  range  is  to 
be  regarded  as  a  geologic  unit,  formed  at  a  definite  time  by  a  set  of 
processes  operating  toward  this  end. 

A  series  of  ranges,  independent  of  one  another,  but  formed 
approximately  at  the  same  time  during  a  given  geologic  period  and 
having  a  common  general  trend,  is  known  as  a  mountain  system. 
So  in  the  Rocky  Mountains  a  series  of  ranges  constitutes  what  Dana 
has  termed  the  Laramide  Mountain  system,  while  in  the  eastern 
United  States  the  Appalachian  Range,  running  through  Pennsyl- 
vania southward,  the  Acadian  Range  of  Nova  Scotia  and  New 
Brunswick,  and  the  Taconic  Range  of  western  Massachusetts  to- 
gether constitute  the  Appalachian  System. 

A  combination  of  mountain  systems,  such  as  those  of  the  Andes, 
constitutes  a  cordillera.  Thus  the  whole  vast  mountainous  region 

373 


374  TEXT-BOOK   OF   GEOLOGY 

extending  from  the  eastern  front  of  the  Rocky  Mountains  to  the 
Pacific,  and  from  Mexico  northward  through  the  United  States  and 
Canada  into  Alaska,  with  its  various  chains,  systems  and  ranges, 
such  as  the  Rocky  Mountains,  the  Sierra  Nevada,  the  Cascade  and 
Coast  Ranges,  etc.,  is  collectively  known  as  the  North  American 
Cordillera. 

Origin  of  Mountains.  —  There  are  three  different  kinds  of  agen- 
cies to  which  mountains  owe  their  origin  and  these  are  igneous 
agencies,  erosion,  and  movements  of  the  earth's  crust.  As  we  shall 
see  later,  no  sharp  line  can  be  drawn  between  the  different  moun- 
tain forms  produced  by  these  three  agencies,  but  for  the  sake  of 
convenience  and  illustration  we  may  distinguish  the  following  types. 

Mountains  Formed  by  Igneous  Agencies.  —  These  may  be  sub- 
divided into  two  classes.  In  the  first  the  elevations  have  been  pro- 
duced by  extrusion  of  material,  and  these  are  illustrated  by 
volcanoes.  Some  of  the  loftiest  peaks  in  the  world,  like  those  of 
the  Andes  (up  to  23,000  feet)  and  Kenia  (17,400)  and  Kilimanjaro 
(19,700)  in  Africa,  are  of  igneous  rocks.  They  are  situated,  how- 
ever, in  high  plateaus,  for  the  Andes  12,000-14,000  feet,  in  Africa 
about  6,000  feet,  which  accounts  for  a  large  part  of  their  great  height. 
Many  oceanic  islands  are  really  great  volcanic  mountains  seated  on 
the  ocean  bottom  and  rising,  as  in  the  case  of  Hawaii  (30,000  feet) , 
to  tremendous  heights  above  their  base.  See  page  116. 

Mountains  produced  by  intrusion  of  igneous  material  into  areas 
of  stratified  rocks  are  of  the  second  class,  and  would  be  illustrated 
by  laccoliths  (page  316)  and  also  in  part  by  necks  and  stocks 
(pages  318,  319),  although  in  these  cases  the  work  of  erosion 
has  also,  as  a  rule,  played  an  important  role  in  developing 
the  mountain  forms  by  cutting  away  the  softer  sedimentary  ma- 
terial, and  leaving  the  more  resistant  igneous  masses  exposed.  The 
best  examples  of  laccoliths  are  found  in  the  region  of  the  Rocky 
Mountains,  especially  in  outlying  districts  not  far  from  the  ranges 
of  the  main  chains.  Thus  the  Henry  Mountains  in  Utah,  the  West 
Elk  Mountains  in  Colorado,  and  the  Little  Belt  and  Judith  Moun- 
tains in  Montana  are  mountain  groups  produced  by  laccolithic 
intrusions.  In  some  cases,  as  in  parts  of  Wyoming  and  Montana, 
there  are  domed  hills  of  sedimentary  beds,  in  which  no  igneous 
rock  is  visible.  By  analogy,  we  place  these  in  this  same  class,  in 
the  belief  that  the  cover  has  not  yet  been  eroded.  As  we  shall 
see  later,  igneous  intrusion  has  also  helped  in  the  development  of 
many  of  the  great  ranges  made  by  folding. 

Mountains  Formed  by  Erosion.  —  It  may  happen  in  the  general 


MOUNTAIN  RANGES:   THEIR  ORIGIN   AND  HISTORY      375 

erosion  of  an  uplifted  area  of  country  that  some  parts  and  places 
during  the  process  of  lowering  are  left  projecting,  and  these  may  be 
of  size,  sufficient  in  relation  to  their  surroundings,  to  be  designated 
as  mountains.  Some  of  the  buttes  in  the  western  United  States 
(page  36)  are  examples  of  this.  The  Catskill  Mountains  in  New 
York  State  have  been  generally  referred  to  as  a  mountain  group  of 
this  character,  etched  out  by  erosion  from  a  plateau  of  uplifted 
sedimentary  strata.  Looking  from  its  brink  into  the  Grand  Canyon 
of  the  Colorado,  one  sees  a  great  number  of  pointed  remnants  of 
erosion  rising  from  the  depths  of  the  chasm.  If  they  were  removed 
from  this  stupendous  gorge  and  placed  on  a  plain  many  of  them 
would  form  large  mountains,  and  the  aspect  which  they  present,  as 
one  looks  upward  at  them  from  the  bottom  of  the  canyon,  is  that  of 
a  high  and  rugged  mountain  range.  They  have  been  formed  from 
the  dissected  run  of  the  great  plateau,  and  are  therefore  in  the 
same  class  as  the  Catskills.  Thus  from  the  erosive  dissection  of 
uplifted  masses,  or  plateaus,  mountains  may  be  made. 

Such  uplifted  plateaus  may  consist  of  sedimentary  beds  in  horizontal  posi- 
tion, or  of  these  with  associated  sheets  of  lava,  or  they  may  be  made  of 
folded  and  disturbed  strata  which  have  been  previously  peneplaned  by 
erosion.  The  mountain  forms  etched  out  by  erosion  differ  in  the  two  cases. 
In  the  first  they  are  often  flat-topped  or  pyramidal  with  slopes  like  those  seen 
in  Figs.  23  and  34.  In  the  second  they  are  apt  to  be  long  ridges  with  crests 
of  the  harder  rock  strata,  often  in  parallel  groups,  with  gaps  or  notches 
cut  through  them  by  the  older  consequent  master  streams,  whose  tributaries 
drain  subsequent  valleys  between  the  parallel  ridges.  Mountains  thus  formed 
by  the  dissection  of  a  former  peneplain  are  to  be  seen  in  parts  of  the 
Appalachian  Highlands,  as  will  be  seen  later. 

Mountains  Formed  by  Movements  of  the  Crust.  —  The  types  of 
mountains  which  have  been  considered  in  the  foregoing  discussion, 
although  of  geologic  interest,  and  in  some  places  forming  groups  or 
masses  of  considerable  size,  are  relatively  of  small  importance  com- 
pared with  the  ranges  produced  by  movements  of  the  earth's  shell. 
For  it  is  especially  by  this  agency  that  the  great  mountain  ranges, 
which  in  so  many  places  constitute  the  dominating  features  of  relief 
of  the  earth's  surface,  have  been  made.  According  to  the  nature  of 
the  movement  and  its  results,  we  may  divide  many  ranges  into  two 
classes,  one  in  which  they  have  been  made  chiefly,  or  entirely,  by 
dislocation,  or  faulting,  resulting  in  vertical  uplift  of  crust  blocks, 
and  one  in  which  they  have  been  produced  by  wrinkling,  or  folding, 
of  the  earth's  shell.  In  the  former  case  the  ranges  are  the  exposed 
edges  of  great  tilted  blocks  of  the  outer  shell,  and  in  allusion  to  this 


376 


TEXT-BOOK   OF  GEOLOGY 


fact  are  sometimes  called  block  mountains.  In  the  second  case, 
the  structures  made  by  folding  are  typically  anticlines  and  synclines 
(page  296)  and  they  may  be  designated  as  folded  ranges.  But, 
although  we  can  find  good  and  clear  examples  of  these  structural 
types,  on  the  other  hand  in  many  cases,  and  these  include  some  of 
the  greatest  ranges,  like  the  Alps,  both  folding  and  crushing,  that  is 
to  say,  dislocation  or  faulting,  have  worked  together  to  produce 
them.  Sometimes  both  agencies  have  been  operative,  sometimes 
one  has  been  more  important  than  the  other,  and  if  we  add  that 
often  intrusions  of  magma  have  also  aided  in  the  process  it  can 
be  easily  seen  that  mountain  ranges  of  great  structural  complexity 
have  been  produced. 

From  what  has  been  stated  in  the  foregoing,  it  may  now  be  seen 
that  different  types  of  mountains,  according  to  the  agencies  produc- 
ing them  and  their  structural  results,  may  be  summarized  in  tabular 
form  as  follows: 


Agency 


Mode  of  Operation 


Structural  Results 


Igneous 

Erosion 


Movements 
of  the  crust 


Built  up  by  extrusion  of  material. 
Upraised  by  intrusion  of  material. 

Masses  etched  out  and  left  in 
relief. 

Produced    by    dislocation,    or 

faulting,  of  blocks. 
Upraised  by  folding  of  crust.     . 
Combinations  of  above. 


Single,     or     grouped 

mountains. 

Volcanoes,  Lava  domes. 
Laccoliths. 

Dissected  plateaus. 

Mountain  ranges 
Block  mountains. 

Folded  types. 

Complex  structural  types. 


Although,  as  mentioned  above,  clearly  defined  examples  of  each 
of  these  types  are  not  infrequent,  it  is  true  that  all  gradations 
exist  between  them,  and  that  types  of  compound  nature  are  most 
frequent,  as  will  appear  in  the  following  sections. 

Block  Mountains.  —  As  stated,  these  have  been  formed  by  the 
faulting  and  tilting  of  great  blocks  of  the  earth's  crust.  Their 
structure  is  most  clearly  seen  in  regions  of  stratified  rocks  whose 
beds  were  previously  horizontal;  the  tilted  block  then  presents  a 
more  or  less  dissected  fault-scarp  rising  to  the  crest  line  on  one  side 
and  a  much  gentler  slope  on  the  other,  the  beds  dipping  in  the 
direction  of  the  gentler  slope.  More  commonly  the  same  structure 
of  tilted  blocks  occurs  in  regions  where  the  strata  have  been  pre- 
viously folded  and,  perhaps,  injected  with  igneous  masses,  and  then 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     377 

greatly  reduced  by  long  continued  erosion.  In  this  case  the  struc- 
ture is  not  so  directly  evident,  but  may  be  inferred  from  the  nature 
and  arrangement  of  the  block  ridges,  and  from  certain  characteris- 
tic physiographic  features  which  they  exhibit,  such  as  the  straight 
base-line  from  which  the  mountain  slopes  arise  from  the  plain  below, 
and  which  may  cut  off  the  ends  of  the  descending  spurs  with  abrupt 
faces.  This  type  of  mountains  is  illustrated  by  the  north  and 
south  ranges  in  parts  of  the  region  of  the  Great  Basin  in  the  western 
United  States,  lying  between  the  Wasatch  Range  on  the  east  and  the 


West 


Sierra  Nevada 


Basin  Ranges  of  Fault  block  Type 


Wasatch  Range 


Fig.  280.  —  Diagrammatic  section  east  and  west  through  the  Great  Basin,  showing 
how  the  structure  is  dominated  by  faults. 

Sierra  Nevada  on  the  west.  An  east  and  west  section  of  this  is 
diagrammatically  shown  in  Fig.  280.  The  production  of  these 
mountains  by  normal  faulting  has  been  already  alluded  to,  pages 
365  and  369. 

The  structure  here  shown  is  thought  to  be  most  typically  developed  in  the 
northern  part  of  the  Great  Basin.  Between  the  great  wall  of  the  Sierra  to 
the  west,  and  the  Wasatch  on  the  east,  the  country  is  filled  with  many  north 
and  south  mountain  ranges,  which  are  rather  narrow  and  from  10  to  50  miles 
in  length.  They  may  be  readily  seen  in  any  good  atlas  on  the  maps  of 
Nevada,  Utah,  and  adjacent  states.  Between  them  lie  plains  and  in  places 
lakes,  some  fresh  but  others  salt,  the  region  affording  the  most  conspicuous 
examples  of  interior  drainage  in  North  America,  as  previously  explained 
under  salt  lakes.  See  Fig.  294. 


Fig.  281.  —  Diagrammatic  east  and  west  section  across  Triassic  sandstone  and  in- 
cluded sheets  of  trap  rock.  Vertical  scale  (and  thickness  of  trap)  exaggerated. 
CS,  crystalline  schists;  G,  granite;  S,S,  sandstone  and  shale;  heavily  black-lined, 
trap.  Near  New  Haven,  Conn. 

The  history  of  the  Great  Basin  region  does  not  begin  with  the  time  when 
block-faulting  took  place.  Long  prior  to  this  were  periods  of  deposit  of  sedi- 
ment, of  folding  and  faulting  of  strata,  and,  no  doubt,  of  mountain  elevation, 
followed  by  great  erosion,  and,  perhaps,  peneplanation.  It  was  only  after 
this  that  the  block-faulting  began;  as  a  consequence  the  structure  of  the 
ranges  is  complex,  and  in  many  cases  not  yet  worked  out.  The  plains  of 
debris  derived  from  their  erosion,  which  lie  between  them,  as  suggested  in 
Fig.  280,  conceal  the  structure  of  the  areas  beneath  them.  There  is  good 


378  TEXT-BOOK   OF  GEOLOGY 

reason  for  thinking  that  some  of  the  faulting  is  not  only  recent,  but  that  it 
has  continued  into  the  present.  But,  although  the  structure  of  these  ranges 
may  be  complicated,  and  several  agencies  may  have  contributed  to  their 
present  topography,  it  is  true,  in  the  main,  that  the  faulting  of  the  region  and 
the  tilting  of  the  blocks  produced  by  it,  have  been  the  dominant  agents  which 
have  given  rise  to  most  of  these  ranges. 

Block  mountains  made  by  faulting  and  erosion  occur  also  in  other  parts  of 
the  world,  and  many  such  examples  could  be  quoted.  Thus  they  are  found 
in  south  Norway,  where  in  places  heavy  sheets  of  igneous  rock  have  pre- 
served the  tilted  sediments  beneath  them  from  erosion  and  form  the  crests 
and  backs  of  one  side  of  the  ridges.  The  same  structure  is  repeated  on  a 
small  scale  in  the  sunken  area  of  Triassic  sandstones  running  northward  from 
New  Haven,  Connecticut,  to  Springfield  in  Massachusetts.  Here  also  heavy 
sheets  of  igneous  trap-rock  have  resisted  the  erosion  and  form  the  crests  and 
backs  of  the  ridges,  as  may  be  seen  in  the  accompanying  figure,  281. 

Folded  Mountain  Ranges 

Introduction.  —  All  the  great  mountain  ranges  of  the  world  be- 
long in  the  classes  of  folded  or  of  complex  structural  types.  In 
North  America  the  Appalachian  Mountains,  the  Ouachita  Range  of 
Arkansas,  the  Coast  Range  bordering  the  Pacific,  and  in  Europe, 
the  Jura  Mountains  in  Switzerland,  are  good  examples  of  folded 
ranges,  whereas  the  Rocky  Mountains,  the  Alps,  the  Urals,  the 
Himalayas,  and  the  old  mountains  of  Scotland  and  Norway  present 
illustrations  of  the  complex  types.  It  is  especially  in  these  kinds  of 
mountains  that  the  greatest  exhibitions  of  geologic  phenomena  are 
seen  and  the  lessons,  which  .geology  as  a  science  teaches,  may  be 
learned.  If  one  desires  to  know  the  history  of  a  region,  one  turns 
naturally  to  its  mountain  ranges,  for  here  may  be  found  the  up- 
turned and  dissected  strata,  a  study  of  whose  kinds,  thickness,  and 
fossils  throws  light  upon  past  events,  while  their  foldings  and  dis- 
locations show  the  nature  and  results  of  those  great  dynamic  agen- 
cies, which  from  time  to  time  have  operated  upon  the  outer  portion 
of  the  earth,  and  given  to  it  the  broad  distinctive  features  which 
characterize  it  to-day.  They  are  also  the  theaters  in  which  many  of 
the  forces,  which  are  now  modifying  the  surface  of  the  earth,  play 
their  most  active  roles,  and  we  can  there  see  the  work  of  erosion,  as 
carried  on  by  water  in  its  varied  forms  of  rain,  frost,  snow,  ice, 
streams,  glaciers,  etc.,  most  extensively  shown.  In  most  cases,  the 
making  of  the  great  ranges  has  been  accompanied,  in  addition,  by 
igneous  activity,  and  they  have  been  the  seat  of  intrusions  and  ex- 
trusions of  molten  magmas  which  have  added  their  quota  to  the 
masses  of  material  and  to  the  complexities  of  structure  that  the 
ranges  present.  It  is  by  reason  of  these  things  that  they  offer  prob- 


MOUNTAIN  RANGES:   THEIR  ORIGIN   AND  HISTORY      379 

lems  of  the  highest  interest  and  importance  to  geological  science, 
and,  therefore,  merit  most  serious  consideration. 

Divisions  of  Mountain  History.  —  The  treatment  of  the  subject 
logically  begins  with  the  folded  ranges,  for  they  are  the  most  simple, 
and  then  proceeds  to  a  consideration  of  the  more  complex  types. 
Their  history  most  naturally  divides  itself  into  three  portions,  as 
follows: 

a,  The  pre-orogenic  period,  in  which  processes  and  their  results 
are  preparing  the  place  and  material  for  the  future  range;  b,  the 
orogenic  period  (see  page  241)  in  which  the  range  is  made;  and  c,  the 
post-orogenic  period,  during  which  the  range  has  been  subjected  to 
various  modifications,  chiefly  those  of  erosion,  which  have  brought 
it  to  its  present  condition.  It  must  be  clearly  understood,  however, 
that  no  exact  boundaries,  either  of  time  or  of  the  events  occurring, 
can  be  drawn  for  these  periods;  although  they  are  convenient  dis- 
tinctions for  purposes  of  discussion,  they,  in  truth,  merge  gradually 
into  one  another,  so  that  the  whole  sequence,  like  the  profile  of  the 
range  itself,  represents  a  gradual  culmination  and  decline. 

Pre-Orogenic  Period ;  Thick  Strata.  —  The  detailed  examina- 
tions which  have  been  made  of  folded  mountain  ranges  prove  that 
they  are  composed  of  masses  of  very  thick  sedimentary  beds,  whose 
folding  and  crushing  together  along  a  definite  axis  has  produced 
the  elevated  tract  of  country.  The  length  of  this  axis  is  that  of  the 
range,  which  may  be  50  miles,  or  1000  miles,  or  even  more;  the 
breadth  of  the  tract  may  be  up  to  250  miles  or  more.  The  maxi- 
mum thicknesses  of  the  strata,  which  have  been  determined  in  some 
of  the  great  ranges,  are  indeed  enormous;  in  the  Appalachians 
nearly  25,000  feet,  and  as  much  in  the  Coast  Range,  while  in  others 
it  has  been  estimated  as  even  greater.  But  when  traced  away  from 
the  mountain  tract,  the  beds,  which  the  fossils  show  to  be  of  the 
same  period  of  deposition,  thin  out  and  may  even  disappear  entirely, 
to  be  replaced  by  rocks  of  a  different  nature  and  age.  Thus  the 
strata  composing  the  Appalachians  have  thinned  down  in  the  region 
of  the  Mississippi  to  4,000  feet  (4,000  in  Indiana,  5,000  in  Iowa) , 
while  toward  the  Atlantic  to  the  eastward  of  a  line  from  northern 
New  Jersey  southwestward  to  Georgia  and  beyond,  they  are  cut 
off  by  faulting  and  erosion  and  are  entirely  wanting.  It  is  probable 
that  in  some  cases  the  thickness  reported  in  mountain  districts  may 
be  due  in  part  to  the  swelling  of  the  beds  caused  by  the  crushing 
together  which  they  have  experienced,  and  that  the  first  estimates 
(for  the  Appalachians  40,000  feet)  were  thus  exaggerated,  but  the 
fact  stands,  nevertheless,  that  the  strata  involved  are  of  great  thick- 


380  TEXT-BOOK   OF   GEOLOGY 

ness  and  of  the  order  of  magnitude  mentioned  above.  From  this 
and  other  facts  important  conclusions  regarding  the  pre-orogenic 
period  may  be  drawn,  as  discussed  in  the  following  paragraph. 

Preparation  for  the  Future  Range.  —  From  the  general  prin- 
ciples which  have  been  previously  explained,  it  will  be  clear  to  the 
student  that  a  line  of  thick  heavy  sediments  can  be  laid  down  only 
in  a  place  of  one  kind,  along  the  margin  of  a  land  that  is  being 
actively  eroded.  If  the  sediments  were  entirely  of  a  marine  origin, 
if  they  consisted  solely  of  lime  deposits,  of  chalk  and  limestone,  this 
would  not  be  true,  for  they  might  then,  under  favorable  conditions, 
have  accumulated  in  the  open  sea.  But  consisting  as  they  do  in 
the  great  ranges  of  mingled  beds  of  conglomerates  and  sandstones, 
with  shales  and  limestones,  it  is  evident  that  the  seat  of  deposition 
must  be  near  the  coast-line,  either  of  the  land  on  which  continental 
deposits  may  form,  or  of  the  sea-bottom  which  receives  sediments. 
Since,  however,  the  whole  accumulated  mass  of  strata  may  be  20,000 
feet  in  thickness,  or  even  more,  this  must  mean  that  subsidence  of 
the  marginal  region  of  deposits  took  place  as  the  sediments  ac- 
cumulated, for  depths  of  this  nature  are  not  found  already  exist- 
ing next  to  the  land.  The  preliminary  structure,  then,  which  deter- 
mines the  place  of  the  future  range,  is  a  subsiding  trough  into  which 
sediments  are  deposited  from  a  neighboring  land,  or  lands  under- 
going erosion,  until  a  great  thickness  has  accumulated.  This  relation 
between  subsidence  and  deposit  of  sediments  has  been  previously 
discussed,  page  239,  and  it  has  also  been  stated  that  an  elongated 
subsiding  tract  of  this  nature  is  known  as  a  geosyncline,  page  305. 

In  the  Appalachians  the  strata  show  by  their  fossils,  markings  and  structures, 
such  as  have  been  described  under  sedimentary  rocks,  and  by  occasional  beds 
of  coal,  as  well  as  by  the  coarseness  and  characters  of  the  sediments  in  re- 
peated beds,  that  the  deposition  took  place  in  shallow  water,  and  was  partly 
marine  and  partly  continental  in  nature,  and  also  that  the  process  of  subsidence 
was  not  a  steady  gradual  one,  but  interrupted,  with  periods  of  upbuilding  to 
land  surface  and  of  low  uplifts,  producing  various  configurations  of  land, 
swamp,  and  shallow  sea.  The  history  of  these  changes  is  fully  discussed  in 
the  second  part  of  this  work.  That  the  sediments  are  continued  westward 
toward  the  Mississippi  and  beyond,  growing  finer,  sandstones  giving  place 
more  and  more  to  shales,  and  limestones  becoming  more  abundant,  proves  that 
the  seas  and  water  bodies  lay  in  this  direction,  since  the  land  deposits  tend  to 
give  way  to  those  more  characteristic  of  marine  origin.  It  follows  as  a 
necessary  deduction,  that,  as  stated  above,  there  must  have  been  a  land  whose 
erosion  was  furnishing  the  sediments  and  this  land,  to  which  the  name 
Appalachia  has  been  given,  lay  to  the  eastward  of  the  subsiding  trough. 
There  were  also  other  land  areas  to  the  northward  and  westward  of  the 
great  embayment.  Since  Appalachia  was  a  land,  and  being  eroded,  it  could 


MOUNTAIN  RANGES:   THEIR  ORIGIN   AND  HISTORY      381 


not  at  that  time  receive  sedimentary  deposits,  except  local  ones  of  a  con- 
tinental nature,  and  accordingly,  we  find  no  marine  deposits  of  this  geological 
age  upon  it  between  the  eastern  edge  of  the  Appalachians  and  the  Atlantic. 
In  the  adjoining  plan,  Fig.  282,  is  given  the  generalized  outline  of  the  geosyn- 
cline  which  received  the  heavy  deposits  forming  later  the  Appalachians  from 
southern  New  York  to  Georgia,  The  process  here  outlined  continued  until 


ChT 


Lou., 


GULF  OF 

MEXICO 


Fig.  282.  —  Map  showing  the  situation  of  the  Appalachian  geosyncline  and  of  the 
old  land  of  Appalachia.     Ad,  mass  of  the  Adirondacks. 

the  closing  of  the  coal-making  period  of  western  Pennsylvania  and  until  the 
25,000  feet  of  sediments  had  been  deposited,  when  the  orogenic  one  com- 
menced. It  should  be  noted  also  that  during  the  depression  of  the  geosyncline. 
mountain-making  forces  were  active  over  the  western  part  of  Appalachia,  which 
was  then  in  the  condition  of  a  rising  geanticline.  It  was  the  reduction  of 
these  mountains  by  erosion  that  furnished  much  of  the  material  that  filled 
the  westward  lying  geosyncline. 

Similar  processes  have  preceded  the  making  of  others  of  the  great  ranges, 


382  TEXT-BOOK   OF    GEOLOGY 

both  folded  and  complex.  The  Sierra  represents  the  marginal  deposits  of  a 
land  that  lay  to  the  eastward  where  the  Great  Basin  is  now  situated.  After 
the  mountains  were  formed,  their  erosion  produced  the  material  now  seen 
in  the  Coast  Range,  so  that  here  the  mountain-making  was  successively 
transferred  westward  toward  the  Pacific.  In  the  Alps  the  lands  lay  to  the 
northward  and  their  sediments  were  deposited  in  the  sea  to  the  southwest- 
ward,  while  in  the  Caucasus  the  old  lands  were  to  the  southward  and  the 
sediments  were  laid  down  in  seas  stretching  northward  over  Russia.  It  may 
be  thus  accepted  as  a  general  principle  that  on  one  side  or  the  other  of  the 
folded  ranges  lies  an  area  of  much  older  rocks  representing  the  source  of  the 
material  which  composes  them,  and  upon  which,  therefore,  marine  deposits 
of  that  period  are  wanting.  It  may  be  that  the  old  lands  have  been  after- 
wards depressed  and  covered  by  still  later  deposits  which  mask  them,  but  they 
must  still  be  there. 

Orogenic  Period  and  the  Forces  Involved 

The  period  of  relatively  quiet  preparation  which  has  been  dis- 
cussed, of  long- continued  erosion  and  sedimentation  and  slow 
changes  of  level  of  land  surface  and  sea-bottom,  gives  way  to  a 
more  active  one  in  which  the  earth's  outer  shell  yields  to  pressure 
which  displays  itself  by  enormous  thrusting  in  a  lateral  direction, 


Fig.  283.  —  Section  across  the  Santis  Alps,  N.  E.  Switzerland  (after  Heim,  some- 
what modified),  a,  shales,  breccias,  etc.;  b,  massive  limestone;  c,  shales,  thin 
limestones,  etc. 

tangential  to  the  earth  surface.  By  this  thrusting  the  accumulated 
load  of  sediments  is  thrown  into  folds,  crushed  and  mashed  to- 
gether, so  that  the  thickened  mass  rises  and  the  mountain  range  is 
made.  This  constitutes  the  orogenic  period.  The  process  and  its 
results  thus  simply  stated  are  in  reality  very  complicated,  with 
different  phases  and  with  divergent  features  in  different  regions, 
some  of  the  more  important  of  which  demand  separate  consideration. 
We  shall  take  up  first  the  operating  forces  and  then  the  results  pro- 
duced. 

Evidences  of  Lateral  Pressure.  —  That  the  ranges  have  been 
made  by  the  crushing  together  of  the  geosyncline  with  its  burden  of 
sediments  by  forces  acting  in  a  lateral  or  tangential  direction  is 
clearly  evident  by  the  structures  which  they  present.  Thus,  in  the 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     383 

zone  of  most  intensive  folding,  the  folds  not  only  become  closed  so 
that  their  limbs  are  in  contact  (see  page  304),  but  they  are  even 
more  severely  compressed,  with  mashing  of  the  beds,  and  the  pro- 
duction of  very  complicated  structures.  This  is  shown  in  the 
adjoining  section  through  a  portion  of  the  Alps,  Fig.  283.  It  would 


Fig.  284.  —  Layers  of  wax  and  plaster  folded  by  lateral  pressure,  imitating  structures 
found  in  folded  mountain  ranges.  The  thrust  is  from  the  right  and  in  successive 
layers,  from  a  to  e  the  amount  of  shortening  can  be  seen.  Willis,  U.  S.  Geol. 
Surv. 

be  impossible  to  imagine  the  formation  of  such  structures  except 
by  transverse  compression  with  relief  by  upward  movement. 

Experimental  Proof.  —  Again  the  varied  phenomena  of  folding 
shown  in  the  mountains  may  be  imitated  by  lateral  compression  of 
a  sequence  of  artificial  strata  composed  of  layers  of  some  plastic 
substance,  such  as  wax  or  clay,  placed  upon  one  another.  If  these 
are  laid  in  a  firm  trough  or  box,  one  end  of  which  may  be  forced 
inward  by  the  turning  of  a  screw,  structures  are  produced  whose 


384  TEXT-BOOK   OF   GEOLOGY 

character  is  shown  in  Fig.  284.  The  displacements  and  dislocation, 
the  folding  and  faulting  of  the  strata,  produced  in  miniature  by  this 
method,  are  similar  to  those  observed  on  a  great  scale  in  the 
mountain  ranges. 

Cleavage.  —  In  the  discussion  of  metamorphic  rocks  (page  342)  it 
was  shown  that  this  feature  of  rocks,  especially  of  slates,  was  due  to 
great  pressure,  and  that  the  planes  of  cleavage  were  perpendicular 
to  the  direction  of  pressure.  Now  the  rocks  of  the  great  ranges  in 
the  zone  of  intensive  folding  are  not  only  apt  to  be  metamorphic, 
but  also  to  show  cleavage,  being  turned  into  schists  and  slates  ac- 
cording to  their  particular  composition.  This  becomes  more  evi- 
dent as  the  inner  portions  of  the  compressed  masses  are  exposed  by 
erosion.  Observation  shows  that  the  planes  of  cleavage  usually 
stand  at  high  angles,  and  are  not  infrequently  perpendicular,  while 
the  strike  of  the  cleavage  planes  is,  in  general,  more  or  less  parallel 
to  the  axis  of  the  range.  The  direction  of  the  compressive  force, 
thus  indicated  by  the  cleavage,  is  the  same  as  that  shown  by  the 
folding. 

Faulting.  —  It  is  obvious  that  such  extreme  folding  as  occurs 
could  not  take  place  without  frequent  rupturing,  breaking  and  dis- 
placement of  the  strata,  and  consequent  faulting.  We  find,  there- 
fore, that  the  phenomenon  of  faulting  is  very  common  in  mountain 
ranges;  both  normal  and  reverse  faulting  being  found.  And  as  we 
pass  from  consideration  of  the  simpler  folded  ranges  to  those  of 
more  complex  types  the  faulting  becomes  more  pronounced  until 
finally,  as  we  shall  see  later,  it  culminates  in  the  production  of 
thrust-faults  of  enormous  magnitude.  The  small  angle  of  incidence 
of  the  thrust-planes  to  the  horizontal  and  their  trends  parallel  to 
the  axes  of  the  ranges  are  indicative  of  the  lateral  compression,  or 
approximately  horizontal  thrusting,  which  has  produced  them. 

In  summation  then,  we  may  accept  it  as  a  well-grounded  fact  that 
the  folded  ranges  have  been  made  by  the  lateral  shoving,  or  squeez- 
ing together,  of  the  stratified  beds  laid  down  in  geosynclines.  See 
Fig.  285. 

The  diagram  Fig.  285  represents  the  general  case  in  mountain-making  in 
which  land  and  sea  continue  to  occupy  the  same  position  relative  to  one 
another.  Applied  to  the  Appalachians,  however,  one  should  remember  that 
the  sea  in  A  and  B  is  the  interior  sea  which  covered  the  Central  States  and 
was  west  of  the  land,  whereas  in  C  it  is  the  Atlantic,  east  of  the  land.  In  A 
and  B  one  is  looking  south,  in  C,  looking  north. 

Amount  of  Compression.  —  No  better  idea  of  the  magnitude  of 
the  forces  involved,  and  of  the  masses  operated  upon,  can  be  had 


TEXT-BOOK  OF  GEOLOGY 


385 


a 


) 


386  TEXT-BOOK  OF  GEOLOGY 

than  in  considering  the  amount  of  compression  which  investigation 
shows  has  actually  occurred  in  the  making  of  some  of  the  great 
ranges.  In  the  Appalachians,  estimates  of  40  to  50  miles,  and  in 
some  places  even  more,  are  given  for  the  distances  the  original  width 
of  the  strata  in  the  geosyncline  has  been  shortened  by  the  mashing 
together  of  the  mass.  If  some  of  the  more  extreme  estimates  are 
correct,  the  folded  strata  in  Pennsylvania,  if  smoothed  out  like  a 
crumpled  blanket,  would  also  cover  a  considerable  portion  of  Ohio. 
In  the  eastern  Rocky  Mountains  in  British  Columbia,  McConnell 
estimates  an  original  width  of  50  miles  has  been  shortened  by  com- 
pression into  one  of  25.  For  the  Coast  Range  in  California  the 
shortening  is  about  10  miles  according  to  LeConte's  data.  Thus 
in  the  production  of  the  great  folded  ranges  the  breadth  of  the 
geosynclines  has  been  diminished  from  10  to  50  miles,  or  even  more, 
and  in  the  zones  of  intensive  folding  and  mashing  the  reduction  has 
been  one  half  or  more.  If  the  original  strata  were  20,000  feet  thick 
at  these  points,  and,  as  used  to  be  considered  the  case,  the  folds  were 
upright,  the  compressed  material  would  be  double  this  in  thickness, 
and  the  height  of  the  mountains,  disregarding  the  counteracting 
erosion,  would  be  enormous.  Actually,  however,  since  the  folds  are 
mostly  overturned,  and  even  recumbent,  the  increase  of  thickness — 
and  height — though  considerable,  must  be  much  less  than  such  an 
amount. 

Influence  of  the  Positive  Elements  and  Direction  of  Thrust.  — 
The  old  upland  along  whose  margin  the  sediments  have  been  de- 
posited forms  a  positive  element,  or  horst,  in  the  architecture  of  the 
outer  shell.  It  tends  to  rise  as  the  geosyncline  tends  to  sink,  and  as 
it  becomes  eroded  the  stronger  massive  rocks,  igneous  and  meta- 
morphic,  of  which  its  lower  levels  are  composed,  tend  to  rise  toward 
the  surface.  It  thus  becomes  steadily  more  massive  and  resistant, 
a  more  unyielding  block  or  element  in  the  shell.  The  sinking  zone 
of  accumulating  sediment  is  one  of  weakness;  whether  the  sink- 
ing is  the  cause  for  the  accumulation  of  the  sediments,  or  the  result 
of  it,  has  been  previously  discussed.  Finally,  when  the  shell  yields 
to  compression,  the  sediments  are  driven  against  this  more  resistant 
mass  and  are  crumpled  up.  The  result  is  that  the  beds  appear  to 
be  carried  against  the  previous  continental  area,  or  areas,  by  a 
thrust  coming  from  the  direction  of  the  sea.  It  seems  probable  that 
this  is  largely  apparent,  due  to  the  greater  resistance  of  the  horst, 
and  that  the  contraction  is  general,  so  that  the  geosyncline  is  not 
only  narrowed,  but  also  shortened.  Thus  the  general  trend  of  the 
mountain  chains  seems  to  be  determined  by  the  situation  of  the  old 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     387 

lands  which  form  the  positive  elements,  or  resisting  buttresses,  at 
the  time  the  chains  are  formed,  and  the  folded  ranges  may,  there- 
fore, be  said  to  roughly  outline  ancient  sea-coasts. 

Thus  the  Appalachians  from  southern  New  York  to  Alabama  indicate  the 
former  marginal  coast-line  of  the  old  continent  of  Appalachia.  It  is  believed 
that  this  is  true,  not  only  of  the  simple  folded  ranges,  but  of  the  more  com- 
plex as  well,  in  great  measure,  so  that  the  trend  of  the  Alps  has  been  deter- 
mined by  old  land  masses,  parts  of  which  are  now  visible  in  central  France, 
the  Vosges,  the  Black  Forest  and  Bohemia,  and  this  relation  is  indicated  on 
the  outline  map,  Fig.  286. 


Fig.  286.  —  Showing  the  trend  of  the  Alps  and  their  relation  to  the  old  land  masses. 

The  same  relations  are  seen  in  other  ranges,  like  the  Coast  Range  of  Cali- 
fornia, or  the  range  of  the  Caucasus  between  the  Black  and  Caspian  seas, 
where  the  sediments  laid  out  on  the  land  margin  and  on  the  sea-bottom,  the 
latter  now  represented  by  the  level  plains  of  Russia,  were  driven  against  the 
old  land  of  Armenia,  from  which  they  were  derived. 

The  direction  in  which  the  thrust  appears  to  come,  whether  from  the  land 
toward  the  sea,  or  the  reverse,  for  different  ranges,  has  not  been  agreed  upon 
by  geologists.  Thus  Suess  thinks  that  the  thrust  has  spread  outward  from 
old  land  masses  toward  the  sea  basins,  producing  in  the  east  of  Asia  the 
arc-like  ranges,  which,  especially  as  islands  on  its  continental  shelf,  fringe 
the  coast-line.  Others  think  the  thrusts  were,  on  the  contrary,  against  this  an- 
cient land,  called  Angara.  James  Geikie  thinks  that  across  the  North  Pacific 
the  thrusts  were  toward  the  east;  in  Asia  toward  the  sea,  in  Western  America 
toward  the  land.  It  has  been  customary  to  infer  that  the  direction  of  thrust- 
ing is  always  shown  by  the  attitude  of  the  folds  developed,  that  they  tend 
to  lean  over  in  the  direction  to  which  the  thrust  is  pushing  them ;  that  is, 
to  be  overfolds  rather  than  underfolds ;  thus  in  Fig.  232  the  thrust  is  from  left 
to  right.  But,  as  Chamberlin  remarks,  this  is  a  criterion  of  doubtful  value, 
for  the  original  attitude  of  the  beds,  and  the  nature  of  the  thrusts,  have  much 


388 


TEXT-BOOK   OF   GEOLOGY 


to  do  with  the  character  of  the  folding.  As  the  negative,  or  depressed,  seg- 
ments of  the  earth's  shell  tend  to  sink  more  and  faster  than  the  positive  seg- 
ments, or  horsts,  see  page  242,  their  borders  are  crowded  against  the  latter. 
The  horsts,  being  stiffer,  resist  and  push  back,  and  thrust  is  met  by  counter 
thrust.  The  part  that  is  weakest  yields  and  buckles  up,  or  is  shoved  over  the 
other.  The  direction  of  thrust  is  thus  apparently  much  more  one  way  than 
it  really  is. 

Examples  of  Folded  Range  Structure.  —  It  is  evident  from  the 
preceding  discussion  of  folded  ranges,  dependent  upon  the  folding 
and,  to  some  extent,  the  fracturing,  that  varied  types  of  structure 


Fig.  287.  —  Section  SE  and  NW  across  the  Jura  Range,  showing  simple  structure  and 

symmetrical  folding. 

may  be  found  in  them,  some  of  which  may  be  comparatively 
simple,  while  others  may  be  extremely  complicated  in  nature. 
Some  of  these  have  already  been  described  and  illustrated 
by  sections,  but  a  few  other  important  instances  may  be  men- 
tioned. Thus  in  some  cases  the  structure  is  very  simple  and 
the  ranges  are  composed  of  stratified  beds  only,  thrown  into  more 
or  less  regular  anticlinal  and  synclinal  folds.  The  Jura  Mountains 
of  Switzerland,  a  western  member  of  the  Alpine  system  (see  Fig. 
286) ,  have  long  been  considered  as  a  classic  example  of  this,  and  a 
section  through  this  range  is  given  in  Fig.  287. 


Fig.  288.  —  Section  12  miles  long  illustrating  Appalachian  structure  near  Greeneville, 
Tenn.    Slightly  modified  from  Keith  and  Willis. 

The  Appalachians  from  Pennsylvania  southward  present  an  ex- 
ample of  a  much  more  complexly  folded  range;  in  them  folds  of 
various  kinds,  closed,  asymmetrical,  and  overturned,  as  well  as 
faults  and  thrusts,  are  common.  A  portion  of  their  structure  is 
illustrated  in  Fig.  288.  This  complexity  becomes  so  pronounced  in 
many  ranges,  as  in  the  Alps,  by  the  overturning  of  folds  and 
the  successive  driving  of  huge  rock  sheets  over  one  another  by 
thrust-faults,  that  we  can  no  longer  treat  them  conveniently  in  the 
same  class  with  the  simple  folded  ranges,  but  will  consider  them  in 
a  group  by  themselves,  that  of  the  complex  ranges.  It  must  not  be 
understood,  however,  that  the  two  groups  are  sharply  separated  in 
nature,  for  intermediate  examples  may  be  found.  It  is  only  for 
convenience  and  clearness  of  treatment  that  this  is  done. 


MOUNTAIN  RANGES:   THEIR  ORIGIN   AND  HISTORY      389 

Complex  Mountain  Ranges 

In  addition  to  simple  folding,  greater  complexity  may  be  intro- 
duced into  the  structure  of  mountain  ranges  by  two  other  important 
factors.  One  of  these  is  by  the  addition  of  igneous  rock  material 
coming  from  molten  magma,  either  as  intrusions,  or  extrusions,  or 
both,  and  the  other  factor,  as  suggested  above,  is  the  occurrence  of 
fracturing  and  thrust-faulting  in  places  on  such  a  scale  as  to  greatly 
diminish  the  relative  importance  of  mere  folding.  Of  the  three 
factors,  folding,  faulting  (thrusting),  and  intrusion,  the  first  has 
been  sufficiently  treated.  We  should  now  consider  the  other  two. 

Work  of  Igneous  Agencies.  —  Although  the  making  of  mountain 
ranges  by  compression  appears  to  be  independent  of  direct  igneous 
action,  and  in  some  cases  there  are  long  distances  in  them  in  which 
no  igneous  rocks  occur,  as  in  the  Appalachian  Range  in  Pennsyl- 


Fig.  289.  —  Illustrating  the  granite  core  of  a  mountain  range  as  exposed  after  pro- 
longed erosion.  The  strata  on  either  side  of  the  bathylith  are  crumpled  and 
metamorphic. 

vania  and  West  Virginia,  and  in  the  Coast  Range  in  California  and 
Oregon,  it  is  nevertheless  a  very  common  thing  to  find  that,  attend- 
ant upon  the  folding,  there  has  been  an  upwelling  of  magma  from 
below,  which  is  shown  by  the  intrusions,  or  extrusions,  of  molten 
rock,  and  often  of  both,  which  are  so  frequently  found  in  many 
ranges.  The  effect  of  this  is  to  greatly  add  to  the  height  and  size 
of  the  uplifted  mass,  and  to  thus  increase  the  volume  of  the  range. 
Probably  the  most  effective  way  in  which  this  happens  is  in  the 
intrusion  of  great  bathyliths  (see  page  319),,  which  are  usually  com- 
posed of  granite,  into  the  inner,  lower  portion  of  the  range.  A 
granite  intrusion  of  this  nature  only  becomes  exposed  later  by  deep 
erosion,  and  is  then  often  spoken  of  as  the  "granite  core"  of  the 
range.  As  a  result  of  the  intrusion,  combined  with  the  folding  and 
mashing  of  the  strata,  the  latter  are  subjected  to  profound  meta- 
morphic effects,  and  may  be  changed  over  wide  areas  to  gneisses 
and  schists,  such  as  have  been  described  in  Chapter  XIII.  In- 
stances of  this  are  seen  in  some  of  the  ranges  of  the  Rocky  Moun- 
tains' chain,  in  the  northern  Coast  Range,  in  the  Alps,  the  Caucasus, 
and  others  that  might  be  mentioned,  Fig.  289.  The  older  a  range 
is,  the  more  deeply  it  will  be  eroded,  and  the  more  likely  we  are 
to  find  its  rocks  harder,  more  resistant,  and  the  unaltered  stratified 


390  TEXT-BOOK   OF   GEOLOGY 

kinds  to  be  replaced  by  metamorphic  and  igneous  ones.  The 
uprising  of  these  great  domed  surfaces  of  granitic  rock  also  generally 
adds  to  the  elevation  by  carrying  up  the  stratified  beds  upon  them. 
We  shall  have  occasion  to  consider  this  later  when  the  results  of 
erosion  are  discussed. 

Invading  granitic  masses  of  this  character  are  a  marked  feature  of  many 
mountainous  tracts,  such  as  those  of  eastern  Canada,  of  New  England  (in 
the  Green  and  White  mountains),  in  North  Carolina,  and  in  the  Sierra 
Nevada,  for  example,  while  the  Alps,  and  the  Caucasus,  as  well  as  the  moun- 
tains of  Scotland  and  Norway,  can  be  mentioned  as  examples  for  Europe. 

Where  intrusions  of  molten  magmas  have  occurred  they  may  not  only  make 
great  bathyliths,  but  pressing  upward  where  relief  is  found  in  places  and 
belts  of  weakness  caused  by  folding,  fractures,  and  dislocations,  they  may 
form  intrusive  sheets,  laccoliths,  chonoliths,  plugs  and  stocks,  or  bosses.  Or, 


Fig.  290.  —  Section  illustrating  intrusions  of  igneous  rock  (black)  in  a  folded  and 
dislocated  mountain  region.     Gr,  edge  of  a  granite  bathylith. 

attaining  the  surface,  they  may  extrude  as  lava  flows,  often  of  great  extent, 
and,  if  the  physical  conditions  are  right,  give  rise  to  volcanic  action  with 
the  production  of  cones  of  large  volume  composed  of  tuffs  and  breccias.  The 
Rocky  Mountains  in  Colorado,  Wyoming,  and  Montana  are  a  striking  example 
of  this,  and  during  the  erogenic  period  were  the  scene  of  great  volcanic 
activity  which  became  especially  pronounced  toward  its  close,  when  many 
groups  of  active  volcanoes  existed.  This  igneous  phase  lasted  long  into  the 
post-orogenic  period  and  its  dying  remnants  are  still  seen  in  the  Yellowstone 
Park.  As  a  result  of  the  folding  and  faulting  of  strata,  and  the  intrusion  and 
extrusion  of  magmas,  there  were  produced  ranges  with  geologic  structures 
of  wonderful  complexity,  which  are  now  revealed  to  us  by  the  great  dissection 
due  to  long  erosion.  The  same  features  in  greater  or  lesser  degree  are  true 
of  many  other  mountain  ranges,  and  they  are  illustrated  in  Fig.  290. 

Thrust-Faulting;  the  Alps.  —  Under  the  subject  of  faulting  it 
was  stated,  page  366,  that  reverse  faults  occurred  in  mountain  re- 
gions in  a  number  of  cases  on  a  tremendous  scale  and  with  a  fault 
surface  of  very  low  inclination,  sometimes  nearly  horizontal.  An 
illustration  was  given  in  Fig.  276.  Such  faults  are  known  as  thrust- 
faults,  or,  more  simply,  thrusts.  It  is  now  proper  to  consider  them 
in  their  relation  to  mountain  making.  We  can  do  this  perhaps  best 
by  the  study  of  a  particular  case,  that  of  the  Alps. 

According  to  the  Swiss  and  French  geologists,  who  have  made  de- 
tailed investigations  of  the  Alps,  the  history  of  this  great  mountain 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY      391 

system  may  be  summarized  as  follows.  It  begins  with  the  gradual 
submergence  of  an  ancient  land  covering  the  region  of  the  modern 
Alps.  This  old  land  had,  without  doubt,  been  itself  a  mountainous 
tract,  but  long  continued  erosion  had  mostly  worn  the  elevations 
away,  and  reduced  it  to  a  low  country.  It  was  then  composed 
mainly  of  crystalline  rocks,  igneous  and  metamorphic,  with  some 
included  strata.  As  it  sank,  it  was  covered  by  a  series  of  deposited 


North 


South 


Fig.  291.  —  Diagram  illustrating  three  phases  in  the  history  of  the  Alps.     A,  pre- 

orogenic  period:  a,  crystalline  basement  rocks;  61  62  63,  deposited  strata  (Mesozoic) 

in  three  varieties  (facies);  c,  younger  strata  (Oligocene). 
5,  after  the  first  erogenic  movement:    successive  rock  sheets  thrust  northward  are 

seen  overlapping;    d,  conglomerate  (Miocene)  resting  on  eroded  older  formations. 
C,  after  the  second  orogenic  movement  and  great  erosion:  Alps  of  the  present. 
Each  section  is  shorter  through  compression  and  represents  only  a  part  of  the  one 

above  it.     After  Steinmann,  slightly  modified. 

beds,  the  waste  of  lands  to  the  northward,  shales  and  sandstones, 
associated  with  limestones  of  marine  origin.  These  strata  attained 
a  thickness  in  places  of  10,000  to  15,000  feet.  This  was  the  pre- 
orogenic  period. 

The  orogenic  period  begins  with  compression  of  the  area  in  a 
general  north-south  direction,  generating  a  thrust  that  apparently 
was  directed  toward  the  north.  Seemingly,  this  did  not  seriously 
affect  the  crystalline  basement  rocks,  although,  as  suggested,  they 
may  have  risen  in  a  broad  low  dome,  a  geanticline.  But  the  super- 


392 


TEXT-BOOK   OF  GEOLOGY 


imposed  strata  were  thrown  into  folds,  and  over  wide  areas  these 
developed  into  overfolds  which  faulted,  and  the  upper  limbs  were 
driven  forward  as  huge  rock  sheets  for  vast  distances  over  the  thrust- 
surfaces.  It  is  even  thought  that  in  some  cases  the  strata  broke 
and  were  forced  forward  without  folding.  Thus  a  great  series  of 
rock  sheets  were  thrust  northward,  the  later  partly  over  the  earlier, 
reversing  in  places  the  normal  order  in  which  they  had  been  de- 
posited. They  covered  up  the  earlier  crystalline  basement. 

Then  followed  a  period  during  which,  if  crustal  movement  did 
not  absolutely  cease,  there  was  relative  quiet,  and  the  rock  sheets 
rested.  Their  whole  overthrust  masses  were  subjected  to  profound 
erosion  and  in  considerable  part  carried  away.  Especially  along 
the  north  front  of  the  Alpine  mass  this  waste  was  deposited  in  enor- 
mous thicknesses  of  gravels,  which  formed  in  time  heavy  conglomer- 
ates. The  deposits  were  laid  down  in  an  inland  sea  which  covered 
an  area  to  the  north  of  the  mass. 


Fig.  292.  —  Movement  of  a  broken  recumbent  fold  along  a  thrust-plane  A, A:  above 
A, A,  to  the  right;  below,  to  the  left.  B,B,  the  "roots"  of  the  fold  left  behind; 
D,D,  and  C,C,  two  possible  surfaces  of  erosion. 

Next  followed  a  renewed  period  of  compression  and  northward 
and  northwest  shoving.  But  this  time,  instead  of  the  driving  for- 
ward of  rock  sheets,  the  latter  were  thrown  into  folds  and  the  under- 
lying crystallines  were  also  bulged  up  in  great  masses.  In  the 
movement  the  great  conglomerates  were  driven  against  and  dis- 
turbed and  the  outlying  deposits  folded  up,  as  in  the  Jura  Moun- 
tains. The  final  consequence  was  the  forming  of  the  east-west  and 
north-south  ranges  of  the  Alpine  system,  as  we  now  know  them.  See 
Fig.  286.  The  results  of  three  phases  of  Alpine  history  may  be 
seen  in  Fig.  291,  diagrammatically  depicted. 

The  process  of  thrusting  briefly  sketched  above  for  the  Alps,  ap- 
plies also  in  a  general  way  for  many  other  of  the  ranges.  We  know 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     393 

that  it  has  occurred  in  places  in  the  Rocky  Mountains,  in  the  old 
mountains  of  Scotland  and  Norway,  to  some  extent  in  the  Appala- 
chians, and  elsewhere.  The  transference  of  the  rock  masses  is  to 
be  measured,  not  only  in  miles,  but  in  tens  and  scores  of  miles,  even 
up  to  100  or  greater.  Only  as  the  careful  and  patient  study  of  the 
great  ranges  goes  on  are  we  able  to  measure  and  appreciate  its 
amount  and  significance. 

The  manner  in  which  the  thrusting  is  supposed  to  occur  by  the  breakage 
of  a  fold  may  be  illustrated  in  Fig.  292;  if  erosion  should  proceed  to  the 
line  D-D,  a  mass  like  A-D  at  the  right  would  consist  of  older  formations  rest- 
ing upon  younger  ones  and  would  be  spoken  of  as  a  mountain  without  "roots." 
Chief  Mountain  in  Montana,  see  Fig.  276,  is  an  example  of  this,  and  there 
are  many  in  Switzerland,  like  the  Matterhorn. 

It  should  be  stated  that  some  students  of  mountain  structures  do  not  en- 
tirely agree  with  the  interpretation  of  Alpine  history  offered  by  the  European 
geologists,  and  sketched  above;  it  has  however  received  general  acceptance. 

Origin  of  the  Compressive  Forces.  —  When  it  was  believed  that 
the  earth  consisted  of  a  relatively  thin  crust  resting  on  a  highly- 
heated  liquid  interior  it  seemed  easy  to  explain  the  origin  of  folded 
mountains  by  assuming  that  there  was  a  regular  contraction  of 
the  earth's  mass  from  loss  of  heat,  and,  since  this  contraction  was 
greater  in  the  heated  interior  than  in  the  cold  outer  crust,  the 
latter  was  folded  up  as  it  gradually  sank  upon  the  shrinking  core, 
very  much  as  the  skin  wrinkles  upon  a  drying  and  contracting 
apple.  This  view,  for  a  variety  of  reasons  which  have  been  stated, 
we  can  no  longer  hold ;  the  earth  behaves  like  a  relatively  solid,  rigid 
body ;  it  cannot  be  wholly,  or  even  largely,  fluid  within,  in  the  ordi- 
nary meaning  of  the  word,  and,  except  locally  or  to  a  superficial 
depth,  it  may  not  be  hot,  at  least  in  any  such  sense  that  it  could 
experience  the  notable  contraction  from  loss  of  heat  demanded  for 
the  origin  of  the  folded  ranges. 

The  evidence,  however,  that  these  ranges  have  been  formed  by 
lateral  compression,  acting  with  tangential  thrusting  movement,  is, 
as  has  been  shown,  direct  and  positive ;  we  cannot  conceive  of  forces 
acting  in  any  other  way  which  would  yield  such  results.  Since  the 
contraction  in  a  horizontal  sense  is  evident,  it  is  for  us  to  find  an 
explanation  of  it ;  is  it  due  to  contraction  in  a  vertical  sense,  that  is, 
radial  contraction  of  the  earth  as  a  whole,  or  may  it  be  explained  in 
some  other  way?  The  volume  of  the  earth  is  so  vast,  that,  in  con- 
sidering this  question,  we  are  liable  to  become  confused  by  the  mul- 
tiplicity of  the  processes  which  affect  it,  but  by  regarding  smaller 
objects  whose  dimensions  and  properties  we  can  hold  more  easily  in 


394  TEXT-BOOK   OF   GEOLOGY 

mind,  the  only  way  in  which  we  can  logically  conceive  of  the  surface 
of  a  spherical  mass  contracting  is  by  a  lessening  of  the  volume  of  the 
object  as  a  whole,  and  by  simple  analogy  we  must  imagine  this  to  be 
the  case  with  the  earth. 

If,  then,  we  are  required  to  rely  on  contraction  of  mass  as  the  ulti- 
mate cause  for  the  contraction  of  surface  area,  which  produces  the 
lateral  pressure,  it  is  possible  to  conceive  of  this  as  occurring  from 
different  causes,  either  mechanical  or  chemical.  They  may  be 
briefly  discussed. 

Mechanical  Causes  of  Shrinkage.  —  Probably  the  view  which  is 
most  commonly  held  to  account  for  contraction  is  the  loss  of  heat. 
This  is  the  survival  of  the  idea  mentioned  above  as  held  when  it  was 
believed  the  earth  was  liquid  within,  but  changed  to  accord  more 
nearly  with  later  knowledge.  It  supposes  the  earth  to  be  solid  and 
rigid  but  very  hot  within  (see  page  261)  and  by  the  progressive  loss 
of  this  heat  the  shrinkage  to  be  caused.  We  cannot  say  positively 
that  this  view  is  untrue,  but  if  several  essential  facts  be  taken  into 
account  it  seems  improbable.  These  facts  are  as  follows: 

If  we  introduce  quantitative  elements,  as  first  suggested  by  Dut- 
ton,  it  is  clear  that  every  mile  of  shortening  of  an  arc  of  the  world's 
surface  requires  a  shortening  of  the  diameter  of  3.i416  mile,  since 
this  is  the  relation  of  circumference  to  diameter,  or  in  round  numbers 
a  shortening  of  the  radius  of  one  mile  shortens  the  circumference  six 
miles.  To  make  the  ranges  now  existent  it  used  to  be  thought  that 
a  shortening  of  the  radius  from  20  to  40  miles  at  least  from  Cam- 
brian time  to  the  present  was  necessary  to  give  the  requisite  contrac- 
tion and  surface  reduction,  but  the  more  recent  work  of  Heim  and 
other  geologists  in  the  Alps  and  elsewhere  shows  that  these  figures 
must  be  greatly  increased.  Heim  now  thinks  that  the  original 
Alpine  area  before  folding  was  from  400  to  750  miles  broad  and  that 
this  has  been  reduced  to  100  miles  in  the  crushing  together.  The 
reduction  for  the  loss  of  a  given  amount  of  heat  has  been  calculated 
and  the  results  prove  that  to  accomplish  the  necessary  shrinkage  a 
very  improbable  loss  of  heat  would  have  had  to  occur.  Moreover, 
every  explanation  for  the  cause  of  the  contraction  that  is  offered 
must  take  into  account  two  things:  first,  that  both  the  paleontol- 
ogic  study  of  the  life  history  of  the  earth,  and  other  facts  relating 
to  the  formation  of  the  stratified  rocks,  show  very  clearly  that  essen- 
tially similar  conditions  of  land  and  water,  of  atmosphere  and 
climate,  have  obtained  upon  it  for  an  immense  period  of  geologic 
time,  during  which  most  of  the  great  ranges  we  know  have  been 
erected ;  and  second,  that  mountain-making  has  not  been  a  regularly 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY      395 

progressive  process,  but  a  periodic  one,  occurring  in  cycles,  inter- 
rupted by  long  periods  of  earth-rest,  as  we  shall  see  more  fully  dis- 
cussed in  the  second  part  of  this  work.  The  simple  loss  of  heat 
alone  does  not  explain  this  latter  fact,  nor  can  we  believe  that  within 
the  historic  period  of  geology  the  outer  shell  of  the  earth  differed 
essentially  in  its  heat  content  from  that  of  to-day.  The  possible 
cause  for  this  thermal  equilibrium  appears  to  be  of  chemical  origin 
and  will  be  discussed  in  a  following  paragraph. 

Of  other  mechanical  causes  suggested,  one  is  the  transfer  of  molten  material 
from  within  to  the  surface,  causing  the  outer  shell  by  its  own  weight  and  that 
of  the  added  material  to  tend  to  sink  in,  and  thus  to  be  under  compressive 
strain.  This  is  a  real  cause  so  far  as  it  goes;  but  it  does  not  go  far.  The 
extrusion  of  1,000,000  cubic  miles  of  lava  would  furnish  but  a  small  fraction 
of  the  shrinkage  required  for  one  of  the  great  ranges. 

Based  on  the  theory  of  isostasy  (see  page  239),  it  is  assumed  by  some,  that, 
as  the  heavier  segments  of  the  earth  sink,  matter  is  crowded  toward,  and 
against,  the  rising  positive  segments,  or  horsts,  and  that  this  flowage  produces 
superficially  a  wrinkling  of  the  outer  crust  which  gives  rise  to  mountain 
ranges.  The  work  of  Barrell,  however,  although  it  shows  that  the  mathe- 
matical evidence  proves  the  existence  of  regional  isostasy,  just  as  clearly  dis- 
proves the  latter  as  a  cause  of  folding.  Other  causes  have  been  suggested, 
such  as  the  transfer  of  heat  from  an  inner  core  to  an  outer  zone,  but  not  neces- 
sarily to  the  actual  surface  whose  temperature  remains  substantially  constant. 
It  has  been  considered  that  no  unreasonable  fall  of  temperature  of  the  inner 
core  would  produce  in  the  outer  shell,  which  receives  heat  faster  than  it  loses 
it  from  the  surface,  an  expansion  yielding  the  results  demanded;  this  could 
not  be,  of  course,  a  process  continuing  indefinitely,  but,  according  to  the  view, 
is  still  in  operation.  Still  another  view  suggests  the  condensation  of  matter 
toward  the  center,  with  a  growing  density  of  the  mass,  and  a  consequent 
lessening  of  its  volume.  Such  views  are,  however,  purely  hypothetical  and 
their  correctness  we  are  able  neither  to  affirm  nor  deny.  The  last  one,  how- 
ever, bears  so  closely  on  the  chemical  explanation  given  below  that  it  seems 
a  necessary  consequence  of  it. 

Chemical  Cause  of  Shrinkage.  —  We  have  had  occasion  in  several  places 
to  refer  to  the  part  which  the  chemical  disintegration  of  matter  may  play  in 
geologic  processes.  Some  of  the  elements  are  known  to  decompose  into  others, 
in  part  gaseous  ones.  Thus  uranium  is  held  to  decompose  into  products  of 
which  lead  is  one  and  helium,  a  gas,  is  another.  With  the  escape  of  the 
volatile  substances,  some  of  which,  like  helium,  would  probably  not  be  held 
by  the  earth  but  would  pass  off  into  space,  there  could  be  a  condensation  of 
volume.  It  may  be  that  this  property  of  matter  is  much  more  general  than 
has  been  supposed,  and  occurs  on  a  scale  which  would  make  it  applicable  to 
the  present  problem.  But  the  process  of  such  disintegration  is  attended  by 
the  production  of  a  relatively  enormous  amount  of  heat,  and  if  we  imagine 
it  as  occurring  on  such  a  scale  that  it  becomes  the  factor  in  the  making  of 
mountain  ranges  by  contraction,  we  must  also  deal  with  the  heat  generated. 
We  might  be  forced  to  consider  the  earth  as  growing  hotter  instead  of 
cooler,  whereas,  from  early  geologic  times,  the  outer  shell  appears  to  have 


396  TEXT-BOOK   OF   GEOLOGY 

been  in  thermal  equilibrium.  But  if  elements  can  disintegrate,  it  may  be 
that  under  proper  conditions,  such  as  might  obtain  in  the  centrosphere,  they 
can  combine,  and  heavier  be  made  out  of  lighter.  This  would  entail  con- 
traction of  volume  and,,  probably,  absorption  of  heat.  These  views  are, 
however,  purely  speculative.  All  we  can  safely  say  at  present  is  that  the 
geological  evidence  seems  to  indicate  a  progressive  condensation  of  the  cen- 
trosphere. 

General  Summary.  —  From  the  discussion  given  above  as  to 
the  ultimate  origin  of  erogenic  forces,  it  is  clear  that  at  the  present 
time  we  are  not  able  to  draw  a  definite  conclusion,  or  one  that  would 
meet  with  general  acceptance.  It  seems  reasonably  safe  to  say 
that  they  have  been  caused  by  contraction  of  the  mass-volume  of 
the  earth,  and  that  it  is  probable  that  this  contraction  is  not  a  simple 
process,  but  due  to  a  variety  of  causes  whose  complexity  we  are  only 
beginning  to  appreciate.  This  is  the  more  evident  when  we  see  that 
the  orogenic  process  has  not  been  a  regularly  progressive  one  but 
periodic  and,  perhaps,  rhythmic,  in  its  operation.  If  the  outer  shell 
were  sufficiently  rigid  to  withstand  the  contractive  strain,  we  might 
understand  this  periodicity  as  the  times  of  yielding  to  accumulated 
stress,  but  since  we  see  this  layer  constantly  yielding  to  it,  with  the 
production  of  faults,  earthquakes,  subsidences,  etc.,  it  is  clear  that 
it  cannot  be  always  yielding  to  stress  and  accumulating  it  at  one  and 
the  same  time.  The  cause  of  this  periodicity  is  at  present  a  mystery, 
which  it  may  be  hoped  further  scientific  knowledge  will  unveil. 

Origin  of  Block  Mountains.  —  The  folded  ranges,  as  we  have 
seen,  are  due  to  the  crushing  of  thick  deposits  laid  down  in  those 
broad,  concave  sinking  tracts  which  have  been  termed  geosynclines. 
The  resultant  crushed  mass  has  been  appropriately  called  a  synclin- 
orium  by  Dana,  that  is,  mountains  formed  in  a  syncline.  In  a 
similar  way  we  can  imagine  the  earth's  crust  warped  into  broad 
upward  folds,  or  domes,  of  relatively  low  uplift,  to  whieh  the  name  of 
geanticlines  has  been  given.  If  a  dome  of  this  character  is  rela- 
tively small  it  may  be  able  to  sustain  itself,  but  if  of  very  wide 
extent,  several  hundred  miles  broad,  and  thus  corresponding  to  the 
great  geosynclines  in  size,  it  may  be  unable  to  do  so,  and  the  arch 
may  break  down,  with  the  production  of  normal  faults  and  tilted 
blocks.  Whether  we  regard  such  a  movement  as  produced  by  con- 
traction of  the  crust  with  inflowage  of  matter  beneath  it  and  conse- 
quent uprise,  or  as  a  rising  of  a  positive  element  or  horst  (see  page 
242)  according  to  the  theory  of  isostasy,  is  largely  a  matter  of 
definition,  the  result  is  the  same.  If  the  dome  remains  unbroken  it 
forms  a  plateau  which  erosion  may  carve  into  mountains;  if  faulted 


MOUNTAIN  RANGES:  THEIR  ORIGIN  AND  HISTORY     307 


and  broken,  the  results  may  be  like  those  seen  in  the  Great 
and  previously  described. 


The  Black  Hflb  of  South  Dakota,  and  on  a 
Mountains  of  Montana,  lepnaseiit  domed  uplifts 
to  mpfrMi?  themselves.    The  strata  on  each  uplift 


,V 


scale  the  Little  Rocky 
have  been  competent 
be 


Fie.  293.  — Section  through  the 


thick,  such  as  would  accumulate  in  a 

have  been  largely  removed  by  subsequent  erosion,  and  are  found  as 

ridges  about  the  flanks  of  the  dome,  see  Fig.  293.    Tne  rising  of  the  arch 

tends  to  lower  the  local  preaaue  and  to  be  accompanied  by  inflow  and,  at 

all  events,  by  upflow  of  material,  which  is  marked  by  the  intrusion  of  igneous 

m*fmm*  as  stocks,  diltM  and  sheets,  and  in  areas  of  weakness  ««H  relief  of 

preaBUie  on  the  borders  as  laccoliths;  these  intrusions  help  to  swell  the 

r*f  t)w>  rmlift:      SUilmiNniiiil-  frtvam  fj 

is  more  or  leas  elliptical  or  even  circular,  into  a  group  of 
valleys. 


r.CL 


TJ.  &  GeoL  Sorr. 


Fig.  294.  —  Eroded  farce  of  a  block 


In  the  case  of  the  Great  Basin,  the  arch,  as 
down,  leaving  the  Sierra  on  one  side  and  the  Wasatch  on  the 
former  abutments;  it  seems  to  have  been  too  vast  to  have  been  able  to  sus- 
tain itself,  see  Fig.  280,  and  the  sunken  blocks  form 
described,  Fig.  29ft.    It  appears,  however,  quite 

ness  of  the  basin,  that  the  arch,  as  such,  never  reafly  existed,  but  that 
blocks  were  formed  during  and  after  the  uplift  by  differential  support, 
not  by  subsidence.    Here,  too.  the  same  upweDing  of  magmas  with 
and  vast  outflows  of  lava  has  occurred.    This  region 


TEXT-BOOK   OF   GEOLOGY 

by  some  to  have  been  subjected  to  great  tension,  or  stretching,  of  the 
crust,  with  breaking  down  into  block  mountains  by  subsidence,  but  it  is 
difficult  to  account  for  such  an  effect  and  it  seems  more  reasonable  to  regard 
it,  and  similar  tracts,  as  incomplete  or  broken-down  geanticlines,  thus  bringing 
them  into  correlation  with  the  general  contractive  movement  of  the  earth, 
and  with  folded  ranges.  It  is  generally  assumed  that  all  of  the  major  faults 
are  normal  in  character,  but  more  detailed  study  of  the  region  may  determine 
some  of  them  to  be  reverse. 

Geological  Date  of  Mountain-making.  —  The  geological  period 
at  which  a  geosyncline  has  been  crushed,  and  the  strata  compressed 
into  a  mountain  range,  is  determined  by  observation  of  the  age  of 
the  latest  strata  involved  in  the  folding,  and  of  the  oldest  beds  rest- 


Fig.  295.  —  Illustrating  determination  of  the  date  at  which  mountain-making  occurs. 
A  are  the  youngest  strata  involved  in  the  folding;  the  folding  is  younger  than 
these;  B  are  not  concerned  in  the  process  and  are  later;  the  folding  is  older  than 
they  are. 

ing  undisturbed  by  mountain-making  processes  upon  the  disturbed 
rocks  about  the  mountain  flanks.  The  date  of  folding  is  obviously 
younger  than  the  folded  or  disturbed  beds  and  older  than  the  un- 
disturbed ones.  This  will  be  clear  from  an  inspection  of  Fig.  295. 
The  accuracy  of  the  method  evidently  depends  upon  the  shortness 
of  the  interval  between  the  times  of  deposition  of  the  two  sets  of 
strata  A  and  B  in  the  diagram. 

The  process  thus  briefly  stated  is  the  general  one  for  the  solution 
of  the  problem,  but  it  is  apt  to  be  more  complicated  than  as  given 
above.  If,  as  is  so  often  the  case,  the  mountains  have  been  formed, 
not  in  one,  but  in  several  periods  of  compression,  it  can  refer  only 
to  the  latest.  Moreover,  erosion  may  have  entirely  carried  away  the 
younger  disturbed  strata  from  their  exposures  in  the  mountains, 
leaving  a  greater  blank  between  A  and  B.  Thus  the  older,  and  in 
consequence,  the  more  eroded  the  mountains  are,  the  more  uncertain 
becomes  the  time  of  their  formation. 

Thus  it  has  been  determined  that  the  Appalachians  were  formed 
for  the  most  part  during  the  Permian  period,  the  Rocky  Mountains 
at  the  close  of  the  Cretaceous  and,  therefore,  later ;  the  Alps  mainly 
at  the  close  of  the  Miocene  period  and  thus  still  later. 

The  Appalachians  were  long  in  making,  the  process  lasting  through  several 
geologic  periods.  The  first  of  the  known  thrusts  came  in  the  Lower  Cam- 
brian, the  second  towards  the  close  of  the  Ordovician,  then  in  the  late  Devo- 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY      399 

nian  came  another,  still  another  near  the  close  of  the  Mississippian  and,  finally, 
the  culmination  in  the  great  thrust  in  Permian  time. 

Summary  of  Orogenic  Period.  —  From  what  has  been  stated  in 
the  foregoing  pages  we  see  that  the  mountain  ranges  which  have 
been  formed  by  movements  of  the  earth's  shell  may  be  due  to 
several  different  processes,  or  to  combinations  of  them.  They  nw 
have  been  made  by  the  simple  dislocation  and  tilting  of  blocks,  by 
folding  without  great  faulting,  or  by  large  thrust  movements,  and  in 
many  cases  by  combinations  of  thrusting  and  folding.  And  along 
with  these  there  are  likely  to  be  injections  and  extrusions  of  molten 
magma,  which  add  their  quota  to  the  mountain  masses.  Thus  while 
simple  types  of  structure  may  be  found,  the  great  ranges,  on  the 
other  hand,  usually  present  very  complex  characters.  Neither 
must  we  also  overlook  the  fact  that  many  of  these  ranges  have 
existed  for  long  periods  of  geologic  time  during  which  they  have 
experienced  at  intervals  powerful  compression  and  thrusting,  as 
already  mentioned  for  the  Alps  and  the  Appalachians.  Such  times 
of  orogenic  pressures  are  intermitted  by  periods  of  rest  during  which 
the  mountains  may  suffer  great  erosion.  This  gives  rise  to  the 
geological  cycles  treated  in  a  following  section.  We  are  thus  led 
naturally  to  consider  what  happens  to  mountains  when  the  orogenic 
forces  cease  operation. 

Post-Orogenic  Processes 

Although  we  may  classify  mountains  by  the  structural  processes 
which  have  aided  in  their  formation,  it  is  not  to  these  processes  alone 
that  the  mountains  as  we  now  see  them  are  due.  For  the  uplift  of 
a  dome,  for  example,  like  that  of  the  Black  Hills,  would  of  itself 
furnish  us  only  with  a  plateau,  not  necessarily  with  mountains. 
Hand  in  hand  with  the  raising  of  the  masses  goes  the  work  of 
erosion,  that  mighty  chisel  of  Nature,  which  shapes  and  carves  them 
into  the  mountain  forms  familiar  to  us.  And  as  by  simple  differen- 
tial erosion  we  see  projecting  buttes  and  rock  spires  left  as  remnants, 
so  it  is  conceivable  to  us  that  this  process  alone,  aided  by  underly- 
ing structure,  might,  from  a  relatively  level  country,  etch  out  forms 
whose  magnitude  would  compel  us  to  call  them  mountains.  The 
residual  mountain  blocks  situated  in  the  trough  of  the  Grand  Can- 
yon are  an  example  of  this,  as  previously  stated. 

The  work  of  erosion,  then,  is  of  the  greatest  importance  in  a  full 
consideration  of  mountains,  it  begins  with  the  first  rising  of  the 
masses,  proceeds  while  the  orogenic  forces  are  at  work,  and  con- 


400  TEXT-BOOK   OF   GEOLOGY 

tinues  long  after  they  have  come  to  rest.  As  its  results  become 
especially  marked  in  this  last  stage,  it  must  be  considered  the  chief 
agent  in  mountain-making  in  the  post-orogenic  period,  and  some  of 
its  more  important  results  may  now  be  considered. 

Earlier  Stages  of  Erosion.  —  So  long  as  the  compressive  orogenic 
forces  are  at  work,  a  mountain  range  will  be  growing,  in  so  far  as 
its  structure  is  concerned.  Whether  it  will  be  actually  rising  in 
height,  or  not,  depends  on  the  adjustment  of  the  varied  forces  at 
work  upon  it,  the  lateral  pressure,  for  example,  in  a  folded  range 
which  tends  to  make  it  rise,  and  the  work  of  erosion  which  tries  to 
cut  it  down.  Always,  during  this  formative  period,  there  is  this 
struggle  going  on,  and  the  range  as  it  exists  at  any  time  is  the 
resultant  between  the  two.  When  the  orogenic  movements  cease, 
then  denudation  has  full  sway  and,  ultimately,  with  the  lapse  of 
sufficient  time  and  provided  no  renewal  takes  place,  the  range  must 
be  cut  down,  base-leveled,  and  extinguished  by  the  ever-gnawing 
erosion.  In  this  process  various  stages  are  to  be  distinguished. 
When  the  range  is  at  its  maximum  of  elevation  then  the  erosive 
agencies  are  most  severe;  to  the  work  of  running  water  on  steep 
slopes  is  added  very  commonly  the  effect  of  frost,  snow  and  ice. 
It  may  happen  also  at  this  time  that  the  rock  material  exposed  to 
erosion  consists  of  the  later  beds  laid  down  in  the  geosyncline,  which 
have  suffered  less  metamorphism  than  the  deeper,  older  ones,  and 
are  thus  softer,  or  less  resistant,  to  erosive  attack.  If  igneous  in- 
jections and  extrusions  have  contributed  to  swell  the  volume  of  the 
range,  it  will  also  be  the  more  easily  eroded  tuffs  and  lavas  that  are 
first  exposed.  Hence,  in  general,  the  outer  material  is  more  easily 
cut  away,  and  with  progressive  erosion  the  inner  core  becomes 
more  and  more  resistant.  Thus,  in  the  early  history  of  a  range  not 
only  is  the  severity  of  attack  of  eroding  forces  apt  to  be  increased 
by  greater  height,  but  they  may  find  less  resistant  material  to  work 
upon.  At  first  the  upraised  masses  begin  to  be  cut  by  the  valleys  of 
the  necessarily  consequent  streams.  The  drainages  thus  appear  upon 
original  slopes  and  continue  to  work  downward  and  backward  into 
the  range.  As  they  do  so  they  begin  to  be  conditioned  more  and  more 
by  the  structures  and  nature  of  the  underlying  rocks.  From  such  a 
youthful  stage,  as  time  goes  on,  the  masses  become  profoundly  graved 
and  rugged,  characterized  by  peaks  and  towering  rock  pinnacles, 
which  alternate  with  deeply  scored  valleys.  The  strongly  notched 
outline  of  such  a  range  at  a  distance  presents  a  saw-toothed  appear- 
ance, which  has  led  to  the  Spanish  name  of  Sierra,  a  saw,  being  given 
to  them.  The  topographic  development  of  the  range  thus  proceeds 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY      401 

from  youth  into  early  maturity,  and  as  erosion  continues  and  the 
valleys  widen,  the  declivities  lessen,  angularities  of  form  tend  to 
disappear,  and  it  becomes  more  and  more  mature.  The  topographic 
forms  of  the  peaks  and  ridges  and  of  the  intervening  valleys  must 
obviously  depend  largely  on  the  nature  and  structure  of  the  rock 
masses  presented  to  erosion.  A  discussion  of  them  would  carry  us 
too  far,  but  some  important  features  are  described  later. 

The  Jura  Mountains  of  Switzerland,  see  Fig.  287,  present  a  type  of  some- 
what youthful  dissection;  here  the  structure  forms  produced  by  folding  still 
cause  the  dominant  topographic  features,  which  erosion  has,  as  yet,  been  un- 
able to  essentially  modify.  Ranges  like  the  Alps,  the  Himalayas,  the  Caucasus 
and  many  of  our  Rocky  Mountains'  system,  are  in  mature  stages  of  dissection, 
although  their  topographic  relief,  that  is,  their  mountain  forms  and  valleys, 
is  dominantly  one  of  erosion,  and  evidently  not  of  folding,  as  in  the  Jura. 
The  words  youthful  and  mature  in  this  connection  must  be  understood  to  be 
merely  relative  terms  and  not  ones  of  absolute  time;  actually  one  range  may 
be  much  older  than  another  and  yet,  on  account  of  its  greater  mass,  difference 
in  material,  or  situation  as  to  climatic  severity  of  erosion,  be  in  a  younger 
stage  of  dissection.  This  has  been  previously  explained  with  respect  to 
river  valleys,  page  51,  and  is  equally  true  here. 


Fig.  296.  —  Illustrating  characteristic  forms  and  outlines  of  late  mature  mountains. 

Later  Stages  of  Erosion.  —  If  erosive  processes  continue  their 
work  of  degrading  a  mountain  mass,  unhampered  by  further  up- 
ward growth  from  erogenic  agencies,  the  range  gradually  passes 
into  a  mature  stage.  As  stated  above,  the  sharp  peaks  and  rough- 
nesses tend  to  disappear,  the  valleys  to  widen  and  open.  And  as  the 
work  continues  it  goes  more  slowly  as  the  inclination  of  slopes 
lessens,  and  as,  in  most  cases,  the  most  resistant  metamorphic  and 
crystalline  igneous  rocks  of  the  inner  core,  or  axis,  are  reached. 
Thus  in  a  range,  very  maturely  dissected,  we  are  apt  to  see  gently 
modeled,  rather  smoothly  rounded  forms  and  outlines,  as  suggested 
in  Fig.  296,  which  contrast  sharply  with  the  angular  features  of  the 
Sierra  type.  And,  as  they  wear  down  more  and  more  and  pass 
gradually  into  old  age,  we  are  apt,  as  stated,  to  find  these  mountains 
composed  of  massive  rocks,  of  schists,  gneisses,  granites,  etc.,  rather 
than  of  the  limestones,  sandstones,  shales  and  lavas,  of  ranges 
in  the  earlier  stages. 

There  appears  to  be  no  well-recognized  term  for  mountains  in  these  later 
stages  equivalent  to  Sierra;  they  are  variously  termed  mature,  subdued,  or 
old  mountains.  Examples  of  them  are  to  be  seen  in  the  mountain  groups 


402  TEXT-BOOK  OF   GEOLOGY 

and  ranges  of  New  England  and  eastern  Canada,  such  as  the  Green  Moun- 
tains, the  White  Mountains,  and  the  Laurentides  of  Quebec,  while  in  Europe 
the  Black  Forest  region  in  Baden,  and  the  old  mountains  of  Scotland  and 
Norway  are  illustrative  of  them. 

Final  Stage:  Peneplanation.  —  Ultimately,  provided  that  no 
new  upwarping  movements  occur,  the  mountains  will  disappear  and 
the  region  which  they  occupied  will  be  nearly  reduced  to  base-level. 
See  page  70 .  Since,  however,  the  process  of  erosion  goes  on  more 


Fig.  297.  —  Stone  Mountain,  De  Kalb  Co.,  Georgia.  A  monadnock,  consisting  of 
granite,  which  rises  above  the  surrounding  plain  of  erosion.  T.  L.  Watson,  Geol. 
Surv.  of  Georgia. 

and  more  slowly  as  the  slopes  lessen,  it  would  evidently  require  an 
enormous  lapse  of  time  to  actually  bring  down  nearly  to  base-level  a 
mountainous  tract,  and  we  have  no  proof  that  this  has  ever  actually 
occurred.  But  we  know  in  some  cases  such  areas  have  been  reduced 
to  a  relatively  low,  almost  featureless  country ;  in  other  words  to  a 
peneplain.  Such  a  country  may  still  be  diversified  by  an  occasional 
hill  projecting  above  the  general  level,  a  residual  of  erosion,  which, 
on  account  of  the  more  resistant  nature  of  the  rock  composing  it, 
or  possibly  its  originally  greater  size,  has  not  been  reduced  like  its 
neighbors.  Such  elevations,  projecting  above  the  surface  of  a  pene- 
plain, have  been  termed  monadnocks,  from  Mount  Monadnock  in 
New  Hampshire,  which  rises  above  the  peneplain  upland  of  central 
New  England.  An  example  is  seen  in  Fig.  297. 

Disregarding  occasional  monadnocks,  we  may  say  that  when  the 
peneplain  stage  is  reached  the  mountains  have  been  obliterated,  but 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     403 

we  may  yet  be  able  to  recognize  the  fact  of  their  former  presence  by 
the  upturned  and  dislocated  nature  of  the  transversely  eroded  strata, 
by  their  frequent  metamorphic  condition,  by  the  slaty  cleavage  and 
faults  which  they  exhibit,  and  by  the  frequent  presence  of  stocks 
and  bathyliths  of  granitic  rocks  intruded  into  them.  We  may  not 
find  the  elevations,  for,  as  LeConte  has  well  said,  "we  find  only 
the  bones  of  the  extinct  mountains,"  but  from  these  remains  we  may 
be  able,  in  imagination,  to  reconstruct  them.  So,  from  the  conditions 
of  the  rocks  of  southern  New  England,  which  is  now  only  a  hilly 
country,  we  are  led  to  infer  that  it  once  presented  the  aspect  of  a 
mountainous  region  with  lofty  ranges  running  through  it,  north  and 
south. 

Re-elevation :  the  Geological  Cycle.  —  From  what  has  been  said 
in  the  foregoing  pages  it  will  be  clear  that  the  life  history  of  moun- 
tain ranges  is  not  necessarily  a  simple  one  consisting  of  a  single 
period  of  compression,  uplift,  denudation,  and  extinction.  On  the 
contrary,  the  more  they  are  studied  the  more  evident  it  becomes 
that  their  history,  in  most  cases,  is  much  more  complicated,  and  that 
the  compressive  orogenic  forces  have  acted  spasmodically,  with  re- 
peated times  of  uplift.  When  a  much  eroded  range  is  again  elevated, 
it  may  be  said,  as  with  rivers,  see  page  72,  to  be  rejuvenated;  the 
erosive  agents  must  again  set  to  work  on  their  task  of  cutting  it 
down.  Thus  a  single  period  of  uplift,  and  then  of  down- cutting 
by  erosion,  may  be  considered  a  geological  cycle,  and  examination 
shows  that  some  ranges  have  passed  through  several  such  cycles. 
The  Appalachians  are  an  example  which  has  been  thoroughly 
studied.  Originally  they  were  formed,  as  has  been  stated,  during 
the  Permian  period.  They  were  then  doubtless  lofty  mountains, 
but  in  succeeding  time  they  were  so  greatly  eroded  that  in  the  Cre- 
taceous period  the  tract  had  been  reduced  to  a  peneplain,  with 
only  here  and  there  occasional  monadnocks  rising  above  it.  After 
this  its  surface  was  again  domed  by  repeated  upswellings  and  the 
revived  rivers  have  carved  it  into  its  present  condition. 

It  must  therefore  be  remembered  that  when  we  speak  of  mountains  being 
young,  mature  or  old,  with  reference  to  their  present  topographic  develop- 
ment by  erosion,  it  is  with  the  present  cycle  of  uplift  that  we  are  dealing. 
Thus  a  range  may  be  young  in  topography  by  rejuvenation  but  old  geologi- 
cally, and  composed  of  ancient  rocks.  This  has  been  previously  pointed  out 
in  the  Alps,  and  it  may  be  suspected  that  many  ranges,  like  some  of  the 
Rocky  Mountains'  system,  which  we  regard  as  young  or  mature  have  really 
been  rejuvenated  from  old  age. 

It  is  through  a  recognition  of  such  repeated  cycles  of  rejuvenation  and 
erosion  that  we  are  able  to  understand  the  problems  presented  by  the  topog- 


404 


TEXT-BOOK   OF  GEOLOGY 


raphy  and  drainage  of  many  regions.  For  example,  we  see  that  master 
streams  have  been  able  to  maintain  their  courses  during  the  time  of  re- 
elevation  and  have  sawed  their  channels  regardless  of  the  underlying  rock 
structure,  so  that  they  are  now  cutting  through  ridges  and  across  the  strike 
of  the  strata  over  hard  and  soft  beds  alike,  instead  of  having  their  courses 
determined  along  weak  belts  as  we  should  naturally  expect,  see  page  76. 


Fig.  298.  —  Illustrating  anticlinal  valleys  and  synclinal  ridges. 

Plateau-forming  Movements.  —  In  connection  with  what  has 
been  said  in  the  preceding  paragraph  it  may  be  well  in  concluding 
the  section  on  mountains,  to  again  bring  to  attention  the  importance 
of  great  plateau-forming  movements,  described  on  pages  375-377, 


Fig.  299.  —  Erosion  of  folded  strata.     North  Beaver  Creek,  Bighorn  Mountains,  Wyo. 
N.  H.  Barton,  U.  S.  Geol.  Surv. 

as  a  factor  in  terrestrial  relief.  In  contrast  with  mountain  ranges 
these  relief  features  have  been  produced  by  vertical  movements, 
and  to  them  the  largest  irregularities  of  the  surface  are  due.  In 
most  regions  marked  uplift  has  taken  place  later  than  folding, 
even  when  the  latter  is  of  recent  geological  age  (Miocene  or  Plio- 
cene) . 

Some  Results  of  Erosion  in  Folded  Strata 

Anticlines  and  Synclines.  —  The  result  of  the  erosion  of  moun- 
tain ranges,  which  commonly  contain  a  variety  of  stratified  rocks, 
folded  and  dislocated  in  different  ways,  and  often  igneous  ones 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     405 

as  well,  is  the  production  of  many  kinds  of  topographic  forms,  de- 
pendent on  the  attitude  of  the  beds  and  on  the  relative  hardness, 
or  resistance  to  erosion,  which  they  exhibit.  Although  some  of 
these  forms  may  be  seen  in  regions  of  low  relief,  they  are,  as  a 
rule,  best  studied  as  a  phase  in  the  history  of  mountain-making. 
One  example  is  found  in  the  erosion  of  anticlines  and  synclines. 
It  would  be  natural  to  think  that  in  a  folded  tract  the  anticlines 
would  be  ridges  and  the  synclines  valleys,  and  in  young  ranges 
this  may,  indeed,  be  the  case,  as  in  the  Jura  Mountains.  But  when 
maturely  dissected  ranges,  such  as  the  Appalachians,  are  studied 
it  is  very  commonly  found  that,  on  the  contrary,  the  valleys  are  cut 
in  the  crests  of  anticlines,  whereas  the  ridges  are  the  bottoms  of 


Fig.  300.  —  Illustrating  the  formation  of  longitudinal  valleys  of  erosion  and 
parallel  ridges. 

synclines.  In  other  words,  the  original  topography,  with  respect 
to  the  structure,  has  been  reversed.  See  Fig.  298.  The  student 
must,  therefore,  remember  that  anticlines  and  synclines  are  terms 
of  structure,  that,  as  previously  explained,  they  do  not  necessarily 
denote  forms  of  topographic  relief. 

The  reason  for  this  appears  to  consist  primarily  in  that  an  anticline's  crest, 
when  in  formation,  tends  to  be  under  tension,  that  is,  to  be  stretched;  conse- 
quently, the  strata  are  apt  to  be  thinned  and  cracked,  producing  a  belt  of  weak- 
ness. The  bottom  of  a  syncline,  on  the  contrary,  is  under  compression,  joints 
and  cracks  become  closed,  and  being  thus  strengthened,  it  can  resist  erosion 
better  than  the  anticline.  In  the  general  lowering  of  the  country  by  denudation 
the  streams  tend  to  carry  away  the  anticlines  faster,  to  seek  out  the  weak  belts, 
and  to  establish  their  valleys  in  them.  See  Fig.  298.  If  the  beds  consist 
of  alternately  hard  and  soft  strata  the  effect  will  be  more  marked,  because 
the  hard  stratum  will  be  cut  through  first  on  the  crest  of  the  anticline, 
where  it  is  highest  and  most  exposed;  the  softer  material  below  being 
reached,  the  erosion  in  it  will  be  more  pronounced  than  elsewhere.  Thus, 
both  by  position  and  structure,  the  anticlines  tend  first  to  be  worn  away,  and 
then  to  become  valleys. 

After  the  Appalachians  had  been  reduced  to  the  Cretaceous  peneplain  and 
then  rejuvenated  by  doming,  it  was  chiefly  along  belts  of  weak  rocks  in 
the  anticlines  that  the  subsequent  streams  established  their  valleys,  Fig.  288. 
In  a  few  cases  where  a  hard  resisting  stratum  has  emerged  through  erosion 
anticlinal  ridges  have  been  formed. 


106        MOUNTAIN  RANGES:  THEIR  ORIGIN  AND  HISTORY 


Fig.  301.  —  A  hogback,  near  Gallup,  New  Mexico.     N.  rf.  Darton,  U.  S.  Geol.  Surv. 


Fig.  302.  —  View  illustrating  the  development  of  topographic  forms  and  drainage  in 
inclined  beds  of  hard  and  soft  strata.  Near  Bisbee,  Ariz.  F.  L.  Ransome,  U.  S. 
Geol.  Surv. 


MOUNTAIN  RANGES:   THEIR  ORIGIN  AND  HISTORY     407 

Parallel  Ridges:  Hogbacks.  —  As  the  erosion  of  folded  strata 
composed  of  hard  and  soft  beds  proceeds,  and  the  hard  beds  are 
broken  through  on  the  anticlines,  drainage  ways  tend  to  establish 
themselves  along  the  belts  of  weak  strata,  as  noted  above.  This 
gives  rise  to  longitudinal  valleys,  following  the  strike  of  the  beds, 
while  the  outcropping  edges  of  the  hard  strata  form  the  crests  of  the 
intervening  ridges,  as  illustrated  in  Fig.  300,  where  several  such 
valleys  on  the  side  of  an  eroded  anticline  have  been  made.  Parallel 
ridges  of  this  nature  are  a  characteristic  feature  over  much  of  the 
eroded  Appalachian  mountain  tract  and  are  found  in  many  other 
districts  of  disturbed  strata.  In  the  foothills  of  the  Rocky  Moun- 


Fig.  303.  —  Diagram  illustrating  the  formation  of  cuestas  and  escarpments. 

tains'  region,  where  the  level  strata  of  the  plains  begin  to  show  the 
initial  foldings  and  displacements  which  culminate  beyond  in  the 
ranges  themselves,  short  eroded  ridges  or  hills  of  this  nature  are  very 
common,  and  are  popularly  known  as  hogbacks.  They  may  be 
several  hundred  feet  high.  An  illustration  of  one  is  seen  in  Fig.  301. 

In  the  erosion  of  a  country  with  the  structure  seen  in  Fig.  300,  the  wear  on 
the  side  of  the  valley  which  exposes  the  outcropping  edge  of  the  strata  is  more 
rapid  than  on  the  slope  made  of  the  backs  of  the  beds.  The  whole  system 
of  drainage  tends  on  this  account  to  move  in  this  direction,  that  is,  to  the 
left,  in  the  figure.  There  is  evidently,  then,  a  shifting,  or,  as  it  is  tech- 
nically called,  a  migration  of  the  divides  between  the  valleys.  If  the  struc- 
ture is  similar  on  both  slopes  of  a  divide,  as  for  instance  in  horizontal  beds 
of  clay,  then  the  erosion  will  be  most  rapid  on  the  steeper  slope,  and  the 
divide  will  migrate  toward  the  gentler  one,  other  conditions  being  equal. 
This  movement  of  divides  is  constantly  going  on  in  a  country  undergoing 
erosion,  its  rapidity  depending  on  the  rate  of  the  erosion,  and  in  the  struggle 
between  divides  new  adjustments  of  drainage  are  constantly  taking  place.  See 
Fig.  302,  and  also  Fig.  21.  This  fact  furnishes  the  key  to  the  solution  of 
problems  regarding  the  origin  of  the  varied  features  of  topography  which 
present  themselves  in  many  places. 

Erosional  Forms  in  Gently  Inclined  Strata.  —  Where  disturb- 
ance of  the  sedimentary  beds  due  to  mountain-making  dies  away  in 
the  level  plains  country,  the  strata  over  wide  areas  may  have  a 
gentle  angle  of  inclination.  The  same  thing  may  occur  also  in  the 
forming  of  a  plateau,  or  in  the  faulting  down  of  broad  masses.  In 


408  TEXT-BOOK   OF   GEOLOGY 

such  cases,  since  strata  are  almost  invariably  composed  of  hard, 
strong,  and  more  resistant  layers  alternating  with  soft,  weak,  less 
resistant  ones,  there  result  from  erosion  topographic  forms  com- 
posed of  a  long  gentle  slope  on  one  side,  with  an  abrupt,  or  even  pre- 
cipitous, descent  on  the  other.  The  long  slope  is  maintained  by  the 
back,  or  upper  face,  of  the  resistant  layer,  while  its  thickness  deter- 
mines the  height  of  the  abrupt  slope  or  cliff,  as  shown  in  Fig.  303. 
Such  an  arrangement  with  long  out-slope  is  known  as  a  cuesta  from 
the  Spanish  name  for  them  in  the  Southwest,  and  a  cliff  of  this  na- 
ture is  often  called  an  escarpment.  Excellent  examples  of  such 
cuestas  are  found  in  the  region  of  the  Rocky  Mountains  and,  on  a 
large  scale,  in  the  Colorado  Plateau  country.  In  some  cases  the 
hard  stratum  may  be  a  sheet  of  lava.  If  the  strata  were  horizontal 
the  escarpment  might  through  erosion  extend  entirely  around  it  and 
we  should  then  have  a  table-land,  or  mesa.  See  page  36. 


CHAPTER  XVI 

ORE  DEPOSITS 

ALAN  M.  BATEMAN 

The  subject  of  ore  deposits  is  a  branch  of  geology  that  deals  spe- 
cifically TKjjth  the  geologic  occurrence,  shape,  content,  and  origin  of 
the  deposits  of  useful  minerals  in  the  rocks  and  gravels.  In  this  re- 
spect it  differs  from  the  art  of  mining,  which  concerns  itself  with  the 
commercial  extraction  of  the  ore  from  the  deposits.  The  two  are 
closely  related,  however,  because  a  knowledge  of  the  geology  of  ore 
deposits  is  necessary  for  intelligent  mining  operations.  Ore  deposits 
are  exceptional  features,  sparsely  scattered  throughout  the  great  mass 
of  rocks.  They  constitute  only  an  infinitesimal  part  of  the  earth's 
crust  but  nevertheless  are  of  great  importance  because  of  the  materials 
they  supply  for  the  use  of  man. 

Ore  Deposits  are  geologic  bodies  that  may  be  worked  commercially 
for  one  or  more  metals.  They  cannot  properly  be  considered  apart 
from  their  geologic  setting;  consequently  the  information  contained 
in  the  preceding  chapters  is  vital  to  an  understanding  of  them.  The 
term  ore  is  often  loosely  used  to  designate  anything  that  is  mined  from 
the  earth;  but  in  a  technical  sense  it  denotes  that  part  of  a  geologic 
body  from  which  the  metal  or  metals  that  it  contains  may  be  extracted 
profitably.  Substances  such  as  coal,  salt,  clay,  or  building  stone,  which 
are  used  practically  in  the  form  in  which  they  are  extracted  from  the 
earth,  are  excluded.  We  thus  connect  ores  with  metals,  and  it  is  in 
this  narrower  sense  that  the  term  is  used  in  this  chapter. 

Materials  of  Ore  Deposits 

The  metal  or  metals  sought  in  an  ore  deposit  are  usually  contained 
in  one  or  more  ore  minerals.  These  in  turn  are  admixed  with  gangue 
minerals,  and  the  mixture,  which  constitutes  the  ore,  is  enclosed  in  the 
country  rock.  In  some  deposits  gangue  minerals  are  absent  and  the 
ore  minerals  are  contained  directly  within  the  country  rock.  For 
example,  the  metal  lead  is  chemically  combined  with  sulphur  in  the 
ore  mineral  galena,  and  the  galena  is  usually  enclosed  in  a  gangue  of 
quartz  or  limestone.  This  mixture  constitutes  the  ore.  After  the 
ore  is  mined,  the  valuable  galena  must  be  separated  from  the  valueless 

409 


410 


TEXT-BOOK  OF  GEOLOGY 


gangue,  this  separation  involving  the  art  of  ore  dressing.  But  the 
galena,  as  such,  is  not  suited  to  the  uses  of  man  and  it  must  be  treated 
further  to  separate  the  desired  metallic  lead  from  its  chemical  com- 
bination with  sulphur.  This  process  involves  the  art  of  metallurgy.  As 
ore  deposits  contain  the  ores  of  nearly  all  the  metals  utilized  today,  it 
will  be  impossible,  in  this  brief  chapter,  to  consider  deposits  of  all  kinds. 
Consequently  it  is  necessary  to  choose  a  few  of  the  more  important 
ones  as  examples  and  concentrate  attention  upon  them.  For  this 
purpose  we  may  select  the  ores  of  gold,  silver,  copper,  lead,  zinc,  and 
iron,  and  illustrate  the  principles  of  ore  deposits  by  means  of  them. 

Ore  Minerals.  —  An  ore  mineral  is  one  that  may  be  used  to  obtain 
one  or  more  metals.  Thus,  galena  is  an  ore  mineral  because  it  is  utilized 
for  its  content  of  metallic  lead;  on  the  other  hand,  feldspar,  which 
contains  aluminum,  is  not  an  ore  mineral,  because  it  is  not  mined  for 
its  aluminum  content.  The  various  metals  are  combined  with  other 
metals  or  elements  to  form  a  great  assortment  of  ore  minerals.  Gold 
usually  occurs  as  the  native  metal,  and  silver  and  copper  are  often  found 
in  that  state.  Most  of  the  other  metals  occur  combined  with  oxygen 
to  form  oxides,  with  sulphur  to  form  sulphides  or  sulphates,  with  ar- 
senic to  form  arsenides,  or  with  both  sulphur  and  arsenic  to  form  sul- 
pho-arsenides;  also  as  carbonates  and  less  frequently  as  silicates. 
Still  other  combinations  are  known,  but  because  of  their  rarity  they 
will  not  be  considered  here.  More  than  one  metal  may  be  combined 
in  a  single  mineral,  as,  for  example,  chalcopyrite,  which  contains  copper 
and  iron,  or  argentiferous  galena  with  both  lead  and  silver.  Also, 
any  one  metal  may  occur  in  several  different  associations;  copper,  for 
example,  occurs  as  a  native  metal,  an  oxide,  a  sulphide,  an  arsenide, 
a  carbonate  and  a  silicate.  The  common  associations  for  the  six  illus- 
trative metals  may  be  more  readily  seen  in  the  following  table,  where  a 
cross  denotes  their  occurrence. 


Metal 

Native 
Metal 

Sulphide 

Oxide 

Carbonate 

Sulphate 

Silicate 

Gold. 

x 

Silver  .  .  . 

x 

x 

Copper  

x 

x 

x 

x 

x 

x 

Lead  
Zinc.  .  .  . 

X 

X 

X 

x 

Iron  

Other  combinations  than  those  mentioned  in  the  table  above  are  of  considerable 
local  importance.  Gold  in  combination  with  tellurium  has  yielded  much  of  the 
wealth  derived  from  Cripple  Creek,  Colorado,  and  both  gold  and  silver  occur  in 


ORE    DEPOSITS  411 

unknown  combinations,  or  else  as  minute  specks  of  native  metal,  in  other  minerals, 
such  as  auriferous  pyrite  or  argentiferous  galena.  Silver  also  occurs  in  combination 
with  chlorine,  giving  rise  to  silver  chloride,  a  form  eagerly  sought  by  the  early  silver 
miners  of  the  West,  and  it  combines  less  commonly  with  arsenic  or  antimony. 

An  ore  deposit  is  not  usually  made  up  entirely  of  one  class  of  the 
combinations  given  above,  as,  for  example,  a  deposit  entirely  of  native 
metals,  of  sulphates  or  of  carbonates.  Most  deposits  contain  repre- 
sentatives of  two  or  more  of  the  groups,  such  as  sulphides,  oxides, 
and  carbonates.  In  addition,  several  metals,  such  as  gold,  copper, 
and  silver,  may  be  mined  from  the  one  deposit,  and  any  one  of  these 
metals  may  occur  in  one  or  more  of  the  combinations  shown  in  the 
table  above.  Thus  it  is  clear  that  the  ore  minerals  of  a  deposit  may  be 
a  complex  mixture  of  several  metals,  occurring  in  several  different  com- 
binations, with  a  single  metal  in  more  than  one  combination.  In 
certain  ore  deposits,  such  as  those  of  iron,  however,  this  is  not  the  case; 
only  the  one  metal,  iron,  is  obtained,  and  this  occurs  in  just  the  one 
combination  —  iron  oxide. 

Certain  combinations  of  individual  metals  are  of  greater  impor- 
tance than  others.  Of  the  many  forms  of  copper,  the  sulphide  is  by  far 
the  most  important,  though  great  quantities  of  the  red  metal  are  ex- 
tracted from  native  mineral,  carbonates,  silicates,  and  sulphates. 
Lead  comes  largely  from  the  sulphide,  but  the  carbonate  contributes 
an  appreciable  amount.  The  zinc  of  commerce  is  derived  mostly  from 
sulphides,  but  the  oxide,  carbonate,  and  silicate  are  of  importance  in 
several  mining  districts.  The  vast  quantity  of  iron  used  in  industry 
is  obtained  almost  entirely  from  the  oxide;  the  sulphide  supplies  no 
iron,  and  the  carbonate  a  very  small  amount.  Most  of  the  gold  out- 
put of  the  world  has  come  from  the  nat  ve  metal,  and  silver  is  pro- 
cured from  both  the  sulphide  and  the  native  metal.  It  must  not  be 
overlooked,  however,  that  other  less  important  combinations,  not  given 
in  the  table  above,  yield  appreciable  amounts  of  the  metals  enumerated, 
as  well  as  many  o  the  minor  metals  not  considered  in  this  chapter. 

The  ore  minerals  may  be  recognized  by  their  physical  properties, 
such  as  color,  luster,  structure,  hardness,  and  relative  weight  for  pieces 
of  the  same  size.  The  properties  of  the  native  metals  are  sufficiently 
familiar  to  everyone.  The  sulphides  are  usually  heavy,  opaque  min- 
erals with  metallic  appearance  and  luster.  Some  oxides  are  metallic- 
looking,  but  most  of  them  are  earthy  or  semi-opaque.  The  carbonates, 
sulphates,  and  silicates  are  not  metallic  in  appearance  but  are  white 
or  colored  and  usually  relatively  soft.  Some  of  the  important  ore 
minerals  from  which  metals  are  extracted  are  listed  below,  and  a  brief 
description  of  them  will  be  found  in  Appendix  A. 


412 


TEXT-BOOK  OF  GEOLOGY 
LIST  OF  ILLUSTRATIVE  ORE  MINERALS 


Metal 

Ore  Mineral 

Composition 

Percentage  of  Metal 

Gold 

Native  gold 

Au   

100 

Silver 

Native  silver 

Ag.  . 

100 

Argentite 

Ag2S  

87 

Native  copper  

Cu.. 

100 

Chalcopyrite 

CuFeS2 

34 

Conner 

Bornite 

Cu5FeS4 

55 

v^vjpjjci  

Chalcocite  

Cu2S  

80 

Cuprite 

Cu2O               .    . 

89 

Malachite 

CuCO3Cu(OH)2  .    . 

57 

Azurite 

(CuCO3)oCu(OH)2  .. 

55 

Galena   

PbS.  . 

86 

Lead   . 

Cerussite  

PbCO3.. 

77 

Anglesite  

PbS04  

68 

Sphalerite 

ZnS 

67 

Zinc 

Smithsonite 

ZnCOs 

52 

Calamine 

H2Zn2SiO5. 

55 

Zincite 

ZnO  

80 

Magnetite  . 

Fe3O4  

72 

Iron  

Hematite  

Fe2O3  '  

70 

Limonite  

2  Fe2O3  •  3  H2O  

60 

Siderite 

FeCO3 

48 

Gangue  Minerals.  —  It  has  been  mentioned  previously  that  an 
ore  deposit  is  made  up  of  a  mixture  of  ore  minerals  and  gangue  minerals, 
which  together  constitute  the  ore.  The  gangue  minerals  are  the  useless 
minerals  associated  with  the  ore  minerals  of  a  deposit,  and  are  usually 
earthy  or  non-metallic  in  character.  In  common  usage  they  are  simply 
referred  to  as  gangue,  an  old  mining  term.  Thus  in  an  ore  deposit 
which  contains  quartz,  calcite,  galena,  native  gold  and  gold-bearing 
pyrite,  the  first  two  are  gangue  minerals  and  the  others  are  ore  minerals 
The  definition  of  gangue  is  not,  however,  an  inflexible  one.  If  siderite 
also  occurred  in  the  same  deposit  it  would  be  a  gangue  mineral,  but 
under  other  conditions  siderite  might  occur  in  sufficient  abundance  to 
constitute  a  deposit  of  iron  ore  and  would  then,  be  an  ore  mineral. 
Gangue  may  be  composed  of  one  mineral  or  a  mixture  of  several,  but 
in  most  cases  it  is  formed  of  common  substances.  It  must  not  be  over- 
looked that  a  gangue  mineral  which  is  useless  today  may  prove  to  be  a 
valuable  ore  mineral  to-morrow.  A  few  of  the  common  gangue  minerals 
are  listed  below: 


ORE    DEPOSITS  413 

Name  Composition. 

Quartz SiO2 

Calcite CaCO3 

Dolomite (CaMg)  CO, 

Siderite FeCO8 

Rhodochrosite MnCO8 

Fluorite CaF2 

Barite BaSO4 

Limonite 2  Fe2O3  •  3  H2O 

Other  gangue  minerals  might  be  mentioned,  but  the  above  will 
serve  as  examples,  and  descriptions  of  them  will  be  found  in  Appendix 
A.  Most  gangue  minerals  are  lighter  in  weight  than  the  ore  minerals, 
and  this  difference  in  weight  enables  them  to  be  separated  and  dis- 
carded in  the  process  of  treatment  of  the  ore. 

Constitution  of  Ore.  —  It  has  already  been  stated  that  there  are  a 
number  of  ore  minerals  and  gangue  minerals  which  occur  in  ore  deposits. 
A  single  deposit  may  contain  a  mixture  of  several  of  these,  but  one, 
two,  or  perhaps  three,  of  the  minerals  may  preponderate  over  the 
others.  In  a  few  deposits  the  ore  minerals  exceed  the  gangue  minerals 
in  volume;  with  most  iron  deposits  a  single  ore  mineral  of  iron,  such 
as  hematite,  will  constitute  the  bulk  of  the  ore.  But  in  the  vast  ma- 
jority of  ore  deposits  the  volume  of  gangue  minerals  greatly  surpasses 
that  of  the  ore  minerals.  Indeed,  the  ore  minerals,  as  a  rule,  form  but 
an  insignificant  part  of  the  ore.  The  relative  proportion  of  ore  and 
gangue  minerals  varies  with  the  kind  of  deposit.  If  one  were  to  go 
through  an  iron  mine,  one  would  see  little  else  but  iron  oxide,  but  in  a 
trip  through  a  profitable  gold  mine  one  might  search  in  vain  to  see  a 
speck  of  gold;  the  gangue  minerals  alone  would  meet  the  eye,  as  the 
gold  forms  but  an  insignificant  part  of  the  ore.  The  mere  fact  that 
iron  is  used  in  great  quantities  and  gold  in  small  quantities  indicates 
their  relative  abundance  in  nature:  also  their  relative  cost  reflects 
their  abundance  in  nature,  for  a  ton  of  iron  may  be  purchased  at  about 
the  same  price  as  an  ounce  of  gold.  In  the  case  of  deposits  of  the 
useful  metals,  such  as  copper,  lead,  and  zinc,  the  relative  propor- 
tion of  ore  minerals  and  gangue  falls  between  the  extremes  of  iron 
and  gold,  but  almost  invariably  the  gangue  predominates.  In  copper 
deposits  the  ore  minerals  are  less  abundant  than  in  zinc  deposits. 
In  a  general  way,  the  relative  selling  price  per  unit  of  metal  is  a  rough 
index  of  the  abundance  of  the  ore  minerals  in  the  ore,  because  the 
higher  the  value  of  the  metal,  the  lower  the  grade  of  ore  that  can  be 
mined  for  the  same  cost.  Thus  the  high-priced  metals,  radium, 
platinum,  and  gold,  can  be  mined  where  the  quantities  are  small,  while 


414  TEXT-BOOK  OF  GEOLOGY 

the  lower-priced  metals,  such  as  lead,  zinc,  and  iron,  necessitate  larger 
volume  and  higher  tenor  to  make  them  workable. 

The  fact  that  ore  usually  consists  of  a  mixture  of  several  kinds  of 
ore  minerals  means  that  a  single  deposit  may  be  worked  for  more  than 
one  metal.  Lead  and  zinc  are  usually  closely  associated  and  both 
metals  are  commonly  won  from  the  same  deposit.  Galena  often  con- 
tains silver,  and  the  two  metals  are  extracted  together.  In  fact,  most 
of  the  silver  produced  in  the  United  States  is  a  by-product  of  these 
ores.  Many  copper  deposits  yield  sufficient  gold  or  silver,  or  both, 
to  add  appreciably  to  the  value  of  the  ore. 

Less  commonly,  a  deposit  may  be  worked  for  a  single  metal.  This 
is  true  of  all  iron  deposits,  of  most  gold  deposits,  and  not  infrequently 
of  copper  and  lead  deposits. 

The  tenor,  or  metallic  content,  of  ore  that  may  be  profitably  mined, 
depends  largely  upon  the  selling  price  of  the  different  metals,  the  size 
and  character  of  the  deposits,  and  their  accessibility.  Obviously  it 
varies  with  deposits  of  different  metals  and  also  with  different  deposits 
of  the  same  metal.  There  is,  of  course,  no  upper  limit  to  the  tenor  of 
ore;  the  richer  the  better.  But  the  lower  limit  is  vital,  since  sufficient 
metal  must  be  extracted  to  pay  for  the  cost  of  producing  it  and  to  yield 
a  profit.  When  the  metals  cannot  be  extracted  profitably  from  material 
it  is  not  ore.  But  what  is  not  ore  today  may,  with  improved  processes 
and  transportation,  be  ore  tomorrow.  A  deposit  containing  $5.00 
worth  of  gold  in  every  ton  of  ore  cannot  be  profitably  worked  in  some 
localities,  but  under  favorable  conditions  ore  containing  $2.00  worth 
of  gold  per  ton,  or  less,  will  yield  a  profit.  The  tenor  of  silver  ore  is 
usually  designated  by  the  number  of  ounces  (troy)  of  silver  contained 
in  a  ton  of  ore;  the  amount  ranges  from  10  to  25  ounces.  Copper  is 
expressed  in  percentage  (1  per  cent  ore  carries  20  pounds  of  metal  to 
the  ton).  A  copper  deposit  containing  the  same  weight  of  metal  per 
ton  as  is  usual  with  silver  ore  would  have  no  value  whatever;  the 
amount  must  be  greater  because  of  the  lower  price  of  copper.  Most 
copper  deposits  must  contain  from  1.5  to  8  per  cent  to  yield  a  profit. 
Under  certain  favorable  circumstances,  as  in  the  Lake  Superior  copper 
deposits,  ore  containing  as  little  as  0.6  per  cent  copper  may  be  treated 
profitably.  At  times  of  high  prices  for  copper,  it  is  possible  to  work 
some  deposits  which  are  unprofitable  when  the  price  of  copper  is  lower. 
Lead  ore  must  contain  from  5  to  10  per  cent  of  lead,  but  if  there  are 
4  to  5  ounces  of  silver  in  the  ore,  the  lead  content  may  drop  as  low  as 
4  to  5  per  cent  and  still  be  commercially  profitable.  Zinc  ore  in  re- 
mote localities  must  contain  from  10  to  30  per  cent  zinc,  but  at  Joplin, 
Missouri,  the  presence  of  a  little  lead  and  favorable  conditions  enables 


ORE    DEPOSITS  415 

ore  containing  as  low  as  3  per  cent  zinc  to  be  mined  with  profit.  Iron 
ore  from  the  Lake  Superior  region  contains  45  to  60  per  cent  iron.  In 
other  localities  the  tenor  may  drop  as  low  as  30  per  cent  iron.  The 
presence  of  gold  or  silver  in  ores  of  the  other  metals  may  allow  ore  of 
lower  grade  than  the  figures  given  above  to  be  worked. 

Origin  of  Ore  Deposits 

Ore  deposits  represent  concentrations  of  unusual  minerals  in  the 
earth's  crust,  and  their  constituents  differ  from  the  minerals  which 
make  up  the  rocks  surrounding  them.  Whence  they  came,  how  they 
were  formed,  and  how  they  were  carried  to  their  present  locations  are 
questions  that  naturally  arise.  As  might  be  expected,  the  many  dif- 
ferent kinds  of  deposits  have  somewhat  different  origins;  nevertheless 
certain  broad  principles  underlie  the  formation  of  most  ore  deposits. 
A  clear  understanding  of  these  principles  is  essential. 

Source  of  Material.  —  The  ultimate  source  of  the  metals  is  deep 
within  the  earth.  They  could  not  have  come  from  the  atmosphere, 
nor  from  the  ocean.  Traces  of  the  metals  are  found  in  the  igneous  and 
sedimentary  rocks  of  the  lithosphere;  but  since  the  sediments  were 
previously  derived  from  the  igneous  rocks,  it  follows  that  the  latter 
must  be  the  source  of  the  minerals,  and  that  they  originally  came  from 
deep  within  the  earth. 

Vehicle  of  Transportation.  —  There  must  have  been  some  agent  of 
transportation  to  carry  the  metals  from  their  deep  source  to  their  pres- 
ent abiding  place  in  the  upper  part  of  the  earth's  crust.  Heated  solu- 
tions or  vapors  could  not  operate  at  that  depth,  for,  as  shown  on  page 
339,  within  the  zone  of  flowage  there  are  no  openings  through  which 
solutions  could  travel.  Molten  rock,  or  magma,  must  have  been  the 
vehicle  of  transportation  which  carried  them  up  from  the  depths  into 
the  zone  of  fracture.  It  is  now  well  established  that  most  igneous  rocks 
contain,  in  addition  to  iron  and  aluminum,  small  quantities  of  the 
different  metals  of  commerce.  But  ore  deposits  do  not  consist  of  such 
minute  particles  of  metals  scattered  in  igneous  rocks,  and  many  of 
them  do  not  occur  in  igneous  rocks,  at  all.  Some  other  forces  have 
operated  to  concentrate  the  metals  into  deposits.  There  are  three 
different  processes  by  which  this  may  have  taken  place:  (1)  The  minute 
particles  of  metals  that  were  carried  upward  by  the  magma  combined 
with  other  elements  to  form  minerals,  and  these  became  segregated 
together  in  the  molten  magma  and  solidified  with  the  igneous  rock. 
(2)  The  metals  were  gathered  from  the  molten  magma  by  highly  heated 
vapors  present  in  it  and  expelled  in  gaseous  form  into  the  rocks  surround- 


416  TEXT-BOOK  OF  GEOLOGY 

ing  the  intrusive,  or  into  the  cooled  upper  portion  of  the  intrusive 
itself.  (3  The  particles  of  metals  were  searched  out  o  the  igneous 
rock,  and  dissolved  by  heated  waters,  by  which  means  they  were  car- 
ried in  solution  into  the  outer  part  of  the  solidified  intrusive  or  into  the 
surrounding  rocks,  where  their  metallic  content  was  deposited  to  form 
ore  bodies.  All  three  of  these  processes  have  operated;  but  the  last 
one  is  by  far  the  most  important,  and  it  is  now  a  well-established  theory 
that  the  greater  number  of  ore  deposits  have  been  formed  by  means  of 
such  heated  waters.  That  they  are  competent  to  dissolve  and  trans- 
port metals  is  shown  by  the  following  facts:  (1)  Warm  water  is  known 
to  be  widely  distributed  through  the  rocks  of  the  earth's  crust,  and 
much  of  it  is  in  slow  circulation;  many  deep  mines  have  hot  waters 
flowing  into  them.  (2)  Water,  if  it  is  heated  or  under  pressure,  and  if 
it  contains  acids  or  alkalies,  is  a  competent  solvent  for  the  minerals  of 
ore  deposits.  (3)  Analyses  of  mine  waters  and  hot  springs  show  that 
they  contain  metallic  compounds  in  solution. 

The  origin  of  these  waters  has  given  rise  to  considerable  discussion. 
Some  have  maintained  that  they  are  of  meteoric  origin,  that  is,  originally 
derived  from  rain,  which,  seeping  into  the  ground,  formed  the  ground- 
water,  and  that  they  then  moved  downward  to  the  warmer  parts  of  the 
earth,  or  else  into  contact  with  magmas,  became  heated,  and  in  this 
state  were  competent  solvents  for  the  metals.  A  few  of  our  ore  de- 
posits, such  as  the  Mississippi  Valley  lead  and  zinc  deposits,  may  have 
been  formed  by  meteoric  waters.  Others  have  claimed  that  the  waters 
are  magmatic  or  juvenile,  that  is,  that  they  have  been  given  off  from 
magmas  and  formed  for  the  first  time.  The  preponderance  of  evidence 
favors  the  latter  view  for  the  majority  of  deposits  It  is  well  known 
that  magmas  do  contain  water,  for  igneous  rocks  have  been  known  to 
expel  water  during  cooling;  and  adequate  evidence  is  also  supplied  by 
the  vast  amount  of  water  vapor  given  off  during  vulcanism.  (Page 
203.)  The  water  from  intrusive  magmas  which  do  not  come  to  the 
surface  cannot  escape  into  the  air  and  must  pass  through  the  cooled 
outer  portion  of  the  intrusive  and  into  the  rocks  surrounding  the  in- 
trusive. Such  waters  are  competent  to  transport  metals  in  solution. 
Some  of  these  waters  reach  the  surface  in  the  form  of  hot  springs,  and 
analyses  show  that  they  contain  metals.  Ore  minerals  of  several 
different  metals  have  been  deposited  at  such  springs. 

Cold  surface  waters  also  have  dissolved  and  transported  metallic  compounds  in 
solution.  Certain  types  of  sedimentary  iron  deposits,  which  will  be  discussed  later, 
are  believed  to  have  had  their  iron  content  so  assembled.  In  the  superficial  alter- 
ation of  ore  deposits,  cold  surface  waters  have  played  an  important  part  as  solvents 
and  agents  of  transportation  for  the  metals.  (Page  435.) 


ORE    DEPOSITS  417 

Deposition.  —  Now  that  the  source  and  the  transportation  of  the 
metallic  compounds  have  been  considered,  it  remains  to  be  seen  how  the 
materials  are  released  from  their  carriers  and  the  ore  and  gangue  min- 
erals formed.  Three  processes  of  transportation  were  considered  in 
the  preceding  section;  in  the  first  one,  the  minerals  segregated  in  the 
molten  magma  and  were  formed  by  simple  solidification  along  with  the 
magma. 

In  the  second  one,  the  metallic  compounds  were  contained  in  highly 
heated  vapors  and  gases.  Two  processes  were  chiefly  operative  in 
bringing  about  the  deposition  of  the  minerals  from  the  gaseous  sub- 
stances: (1)  a  decrease  in  pressure  and  temperature  due  to  contact 
with  cooler  rocks,  resulting  in  the  chilling  of  the  vapors  and  the  depo- 
sition of  the  minerals;  (2)  a  chemical  reaction  between  the  vapors  and 
the  rocks  with  which  they  came  in  contact,  by  means  of  which  the 
minerals  were  deposited.  In  the  chemical  reaction  the  rocks  were  in 
part  destroyed  and  their  places  simultaneously  occupied  by  the  de- 
posited minerals.  For  example,  the  feldspars  of  a  granite  might  be 
attacked  by  the  vapors  and  partly  converted  into  mica,  and  ore  min- 
erals would  be  deposited  where  the  feldspars  originally  existed.  Lime- 
stone, under  such  chemical  action,  would  break  up  into  CaO  and  CO2,* 
some  ingredients  of  the  volatile  compounds  would  unite  with  the  CaO 
to  form  new  minerals,  and  others  would  be  simultaneously  deposited 
in  place  of  the  limestone.  See  also  contact-metamorphism,  page  350. 

In  the  case  of  the  third  and  most  important  agent  of  transportation, 
the  hot  waters  with  their  dissolved  metallic  compounds  would  travel 
through  all  the  available  openings  in  the  rocks.  The  metals  and  other 
compounds  would  stay  in  solution  unt  1  some  new  conditions  were 
encountered  by  means  of  which  they  were  forced  out  of  solution  and  were 
deposited  as  the  minerals  of  ore  deposits.  In  the  larger  openings,  or 
cavities,  as  they  are  called  opportunity  is  afforded  for  such  deposition 
to  take  place,  and  the  cavities  may  become  completely  filled;  if  the 
filling  is  of  sufficient  size  and  richness,  ore  deposits  result.  (Page  424.) 
In  some  places,  only  small  cavities  may  be  filled,  but  if  they  are  suffi- 
ciently abundant  and  close  together  the  whole  may  constitute  an  ore 
body.  The  deposition  is  brought  about  by  several  factors,  of  which  the 
following  are  important:  (1)  the  gradual  lowering  of  the  temperature 
and  pressure  as  the  solutions  traverse  cooler  rocks  more  and  more 
remote  from  their  point  of  emission;  (2)  chemical  reaction  with  other 
solutions  that  may  be  present  in  the  rocks;  (3)  the  effect  upon  the 
solutions  of  the  rocks  which  they  are  traversing;  (4)  the  presence  of 
catalytic  agents;  (5)  electrolytic  action  within  the  solutions.  Of 
these,  change  of  temperature  probably  has  the  most  important  effect. 


418  TEXT-BOOK  OF  GEOLOGY 

Any  one  of  the  factors  is  competent  to  produce  deposition,  but  it  is 
probably  the  result  of  the  combined  action  of  several  of  them. 

Where  there  are  no  cavities  of  appreciable  size,  deposition  may  take 
place  by  chemical  reaction  with  the  rock,  whereby  the  solutions  make 
their  own  cavities  by  dissolving  the  rock  or  portions  of  the  rock,  par- 
ticle by  particle,  and  simultaneously  depositing  equivalent  volumes  of 
ore  minerals  and  gangue.  The  process,  of  course,  can  operate  only  in 
certain  rocks  which  can  be  dissolved  by  the  hot  waters.  This  chemical 
interchange  of  rock  particle  and  ore  particle  is  known  as  replacement, 
and  the  continuation  of  the  process,  until  a  great  volume  of  rock  has 
been  replaced  by  an  equivalent  volume  of  ore,  has  resulted  in  the 
formation  of  large  and  valuable  ore  deposits.  (Page  429.) 

Physical  Conditions  Affecting  Ore  Deposition.  —  Physical  factors 
play  an  important  part  in  the  character  and  location  of  ore  deposition. 
From  the  preceding  sections  it  has  been  seen  that  the  majority  of  ore 
deposits  are  formed  as  a  result  of  magmatic  emanations.  During  the 
earlier  stages  of  the  intrusion,  the  emanations  consist  of  highly  heated 
gases  and  vapors,  as  no  liquids  can  exist  at  the  temperatures  that  pre- 
vail. As  they  move  outward  from  the  magma  and  up  toward  the  sur- 
face, they  pass  through  zones  of  decreasing  pressure  and  temperature. 
The  emanations  then  take  the  form  of  heated  waters,  and  different  ore 
deposition  results,  in  response  to  the  changing  physical  conditions. 
Certain  minerals  form  under  a  wide  range  of  conditions,  but  the  depo- 
sition of  others  is  restricted  to  definite  ranges  of  temperature  and 
pressure,  and  they  are  thus  diagnostic  of  those  particular  conditions. 

At  the  contact  of  the  intrusive  the  temperature  is  high,  and  the  ore 
deposition  formed  by  the  highly*  heated  vapors  and  gases  is  charac- 
terized by  special  mineralogic  associations  and  mode  of  occurrence,  to 
be  discussed  further  under  Contact-metamorphic  Deposits.  With 
lower  temperature  and  pressure  the  magmatic  emanations  will  be  in 
the  form  of  highly  heated  waters  and  will  deposit  different  minerals 
according  to  the  zones  of  temperature  and  pressure  through  which 
they  pass.  Three  such  divisions  have  been  differentiated  by  Lindgren: 
(1)  deposits  formed  at  great  depths  under  high  temperature  and  pres- 
sure; (2)  deposits  formed  at  moderate  depth  by  hot  solutions;  and 
(3)  deposit  formed  at  shallow  depth  by  warm  solutions.  Each  of 
the  groups  contains  certain  typical  minerals,  and  they  differ  from 
each  other  in  the  mode  of  occurrence  of  the  ores. 

The  ore  deposition  of  the  first  division  has  taken  place  at  great  depth,  in  or  near 
bodies  of  igneous  rock  where  very  high  temperatures  and  pressures  exist.  The  ore 
deposits  become  exposed  at  the  surface  only  after  prolonged  erosion  has  swept  away 
the  overlying  rocks.  They  have  been  formed  by  filling  fissures  and  by  replacing 


ORE    DEPOSITS  419 

the  wall  rocks  of  the  openings.  The  minerals  composing  them  are  similar  to  those 
of  the  contact-metamorphic  deposits,  but  quartz  is  more  abundant.  Minerals  that 
form  only  at  high  temperatures,  such  as  garnet,  pyroxene,  amphibole,  tourmaline, 
magnetite,  and  pyrrhotite,  characterize  them.  Gold,  copper,  iron,  tin,  and  other 
metals  are  found  in  these  deposits.  The  tin  and  copper  veins  of  Cornwall,  England, 
are  an  example  of  this  type. 

Deposits  formed  at  moderate  depth  by  hot  solutions  usually  occur  in  or  near  intrusive 
rocks  and  are  exposed  at  the  surface  by  deep  erosion.  During  their  formation,  the 
temperature  ranged  from  150°  to  300°  C.,  and  the  pressure  was  high.  They  have 
been  deposited  in  fissures  or  other  openings,  and  have  also  replaced  certain  soluble 
rocks  to  a  great  extent.  The  "high-temperature"  minerals  are  absent,  and  such 
minerals  as  quartz,  calcite,  siderite,  pyrite,  arsenopyrite,  chalcopyrite,  galena,  and 
sphalerite  are  typical  of  them.  The  deposits  furnish  gold,  silver,  copper,  lead,  and 
zinc,  and  most  of  the  valuable  deposits  of  these  metals  in  the  Rocky  Mountains 
and  California  belong  to  this  group.  The  great  copper  deposits  of  Butte,  Montana, 
are  a  typical  example. 

The  deposits  formed  at  shallow  depths  by  warm  solutions  are  also  commonly  asso- 
ciated with  igneous  rocks,  but  usually  with  extrusive  rocks  or  with  the  small,  shallow 
types  of  intrusives.  They  are  frequent  in  regions  of  comparatively  recent  volcanic 
activity  and  have  been  exposed  by  only  moderate  erosion.  They  have  been  formed 
at  a  temperature  ranging  from  50°  to  150°  C.,  and  at  a  moderate  pressure.  They 
occur  most  commonly  in  fissures  in  shattered  rock,  and  replacement  of  the  wall 
rock  is  subordinate.  They  supply  a  large  part  of  the  gold,  silver,  and  mercury  of 
the  world;  the  lead,  zinc,  and  copper  obtained  from  them  are  of  less  importance. 
The  deposits  are  characterized  by  such  minerals  as  quartz,  chalcedony,  opal,  calcite, 
dolomite,  barite,  fluorite,  native  gold,  marcasite,  argentite,  and  other  more  complex 
minerals  of  silver.  The  "  high-temperature  "  minerals  are  wanting.  The  famous 
Comstock  Lode  of  Nevada  is  an  example  of  this  type,  as  are  also  most  of  the  gold- 
silver  deposits  of  Nevada,  Colorado,  Arizona,  and  Mexico,  and  the  quicksilver  de- 
posits of  California 

Classification  of  Ore  Deposits 

From  the  foregoing  sections  it  has  become  evident  that  all  ore 
deposits  are  not  alike,  nor  are  they  simple  geologic  bodies.  They  are 
formed  by  different  processes,  are  composed  of  numerous  substances, 
occur  in  many  forms,  and  are  affected  by  variable  physical  factors. 
Innumerable  different  kinds  of  ore  deposits  are  the  result,  and  no  two 
of  them  are  alike  in  all  respects;  they  are  as  dissimilar  as  are  the  dif- 
ferent kinds  of  animals  on  the  earth.  In  order  to  understand  ore 
deposits  properly  and  to  consider  them  in  detail,  it  is  necessary  to  sort 
out  the  confused  array  and  to  arrange  them  in  logical  order,  grouping 
similar  ones  together.  This  is  called  classification.  Different  methods 
of  classification  may  be  employed;  if  one  were  to  classify  the  people 
of  the  earth,  one  might  arrange  them  according  to  color  as  white  men 
and  black  men,  according  to  nationality,  or  according  to  stature,  as 
tall  men  and  short  men.  Obviously,  one  grouping  must  be  followed, 


420  TEXT-BOOK  OF  GEOLOGY 

for  to  speak  of  one  group  of  men  as  black  and  another  as  tall,  leads  to 
confusion.  Similarly,  ore  deposits  may  be  classified  according  to  the 
conditions  under  which  they  were  formed,  as  deep  zone  or  shallow  zone 
deposits,  according  to  their  form,  as  veins  or  massive  replacement 
bodies,  on  the  basis  of  their  distribution,  on  the  kind  of  metal  contained 
in  them,  or  according  to  the  processes  which  have  given  rise  to  them. 
Two  or  more  of  these  groupings  may  be  combined  as  sub-divisions. 

For  the  purpose  of  this  chapter,  all  ore  deposits  may  be  divided  into 
two  large  groups,  as  follows : 

/.    Bed-rock  or  Primary  Deposits, 

II.   Disintegration  or  Secondary  Deposits. 

By  this  division  those  deposits  formed  for  the  first  time  by  processes 
of  primary  ore  deposition  within  the  solid  bed-rock  are  distinguished 
from  those  formed  by  processes  of  disintegration  and  erosion  acting 
later  on  the  rocks  and  their  contained  ore  deposits,  and  giving  rise  to 
concentrations  of  ore  in  gravels  or  to  other  products  of  disintegration. 
The  common  mining  terms,  bed-rock,  and  lode  deposits,  refer  to  the 
first  group,  while  placer  deposits  refer  to  the  second  group.  The 
great  variety  of  deposits  previously  referred  to  fall  within  the  first 
large  group,  making  it  necessary  to  divide  the  Bed-rock  Deposits 
further.  Accordingly,  two  major  sub-divisions  have  been  made,  one 
consisting  of  those  deposits  in  which  the  ore  was  formed  at  the  same 
time  and  by  the  same  processes  as  the  rocks  that  enclose  it  (Contempo- 
raneous Deposits],  and  the  other  of  those  in  which  the  ore  was  formed  by 
processes  of  mineralization  acting  long  after  the  rocks  were  formed 
(Subsequent  Deposits).  This  distinction  is  vital,  because  ores  of  the 
former  class  can  occur  only  where  the  enclosing  rocks  occur,  while 
those  of  the  latter  may  be  found  irrespective  of  their  mother  rocks. 
Since  the  enclosing  rocks  of  the  contemporaneous  deposits  may  be 
either  of  igneous  or  of  sedimentary  origin,  so  the  ore  deposits  formed 
contemporaneously  with  the  rocks  must  be  either  of  igneous  or  of 
sedimentary  origin.  This  creates  two  minor  sub-divisions  of  Contempo- 
raneous Deposits :  (a)  Igneous  Deposits,  and  (6)  Sedimentary  Deposits. 

The  group  of  Subsequent  Deposits  includes  so  many  varied  types 
that  it  also  must  be  sub-divided  further.  The  different  processes, 
already  referred  to,  by  which  the  ores  of  this  group  gain  entry  into  the 
rocks,  form  the  logical  basis  of  such  minor  sub-division.  The  heated 
mineralizing  waters,  in  their  journey  through  the  rocks,  may  encounter 
openings  and  cavities  and  fill  them  with  ore,  giving  rise  to  one  sub- 
division, namely,  Fillings  of  Cavities;  the  hot  water  solutions  may 
replace  parts  of  the  rock,  giving  rise  to  another  sub-division,  com- 


ORE    DEPOSITS 


421 


monly  called  Replacement  Deposits.  Again,  the  mineralizing  agents, 
in  the  form  of  highly  heated  gases,  may  attack  and  replace  the  rocks 
adjacent  to  the  intrusive,  giving  rise  to  a  variety  of  replacement  de- 
posits, which  because  of  their  distinct  and  striking  characteristics  are 
considered  separately  under  the  name  of  Contact-metamorphic  De- 
posits. 

The  second  great  group,  the  Disintegration  or  Secondary  Deposits, 
may  be  sub-divided  likewise,  according  to  the  processes  by  which  the 
deposits  were  formed.  The  disintegration  and  concentration  may 
have  been  brought  about  by  the  sorting  action  of  water,  resulting  in 
(a)  Mechanical  Concentrations,  or  Placers,  or  by  chemical  deposition 
at  the  surface,  giving  (b)  Chemical  Concentrations;  or  the  worthless 
material  may  have  been  removed  by  solution,  leaving  the  insoluble 
valuable  materials  behind,  forming  (c)  Residual  Concentrations. 

These  various  groups,  divisions,  and  sub-divisions  may  now  be 
summarized  in  the  form  of  a  table  to  show  more  clearly  the  relation 
of  the  different  deposits  to  each  other,  and  to  the  processes  to  which 
they  owe  their  origin. 

TABLE  SHOWING  CLASSIFICATION  OF  ORE  DEPOSITS 


Ore  Deposits 


I.   Bed-rock  or  primary  deposits 

A.  Contemporaneous  deposits- 

1.  Igneous 

2.  Sedimentary 

B.  Subsequent  deposits 

1.  Fillings  of  cavities 

a.  Fissure  veins 

b.  Other  cavity  fillings 

2.  Replacement  deposits 

3.  Contact-metamorphic  de- 

posits 
II.   Disintegration  or  secondary  deposits 

A.  Mechanical  concentrations 

B.  Chemical  concentrations 

C.  Residual  concentrations 


Processes  of  Formation 


Process  operating  on  bed-rock 
Rock-making  processes 

Process  of  solidification 

Process  of  sedimentation 
Process  of  mineralization 

By  chemical  deposition  from  solution 


By  chemical  replacement  of  rock 
By  contact  metamorphism 

Process  of  surface  disintegration  and 

concentration 

Sorting  action  of  water  or  wind 
Chemical  deposition 
Removal  of  worthless  material 


The  above  classification  is  based  upon  distinct  and  clearly  defined  types  of  ore 
deposits  and  also  upon  theoretical  considerations.  The  process  of  formation  has 
been  emphasized,  because  when  that  is  clearly  understood,  the  mode  of  occurrence, 
the  materials,  and  the  form  of  deposits  become  evident,  and  the  vagaries  of  individual 
deposits  may  be  more  readily  deciphered.  Thus,  if  one  understands  the  process  of 
contact  metamorphism  he  will  realize  that  contact-metamorphic  deposits  will  only 
occur  adjacent  to  deep-seated  intrusives,  that  certain  rocks,  such  as  limestone,  are 
more  likely  to  contain  deposits  of  this  type  than  are  rocks  of  other  kinds,  that  the 


422  TEXT-BOOK  OF  GEOLOGY 

shape  of  the  deposit  will  be  irregular,  and  that  the  minerals  composing  it  will  be  of 
the  high-temperature  type.  Physical  factors,  such  as  temperature  and  pressure, 
will,  of  course,  affect  many  of  the  classes  given  above  and  produce  variations  in  a 
particular  type  of  deposit.  For  example,  a  fissure  vein,  if  formed  under  conditions 
of  high  temperature  and  pressure,  will  partake  of  the  characteristics,  already  pointed 
out,  of  deposits  of  the  deep  zone;  and  one  formed  under  low  temperature  and  moder- 
ate'pressure  will  exhibit  the  features  mentioned  for  deposits  of  the  shallow  zone. 
Such  physical  conditions  may  then  be  said  to  produce  variations  of  some  types 
given  in  this  classification. 

The  individual  classes  given  in  the  table  above  will  now  be  considered 
in  more  detail. 

Contemporaneous  Ore  Deposits 

The  contemporaneous  deposits  are  formed  by  rock-making  processes 
and  are  integral  parts  of  rock  masses.  They  are  simply  portions  of 
rocks,  which,  because  of  their  metallic  content,  happen  to  be  of  value  to 
man.  Were  they  of  no  more  value  than  the  rest  of  the  rock  we  should 
not  think  of  them  as  ore  deposits  at  all,  but  should  regard  them  as  one 
type  of  rock  formation.  As  stated  in  Chapter  X,  rocks  are  divided  into 
igneous,  sedimentary,  and  metamorphic  groups;  and  as  contempo- 
raneous ore  deposits  are  formed  at  the  same  time  as  the  rocks  that  con- 
tain them  they  also  must  be  divided  into  the  same  three  groups.  How- 
ever, an  ore  deposit  which  has  been  metamorphosed  is  often  so  changed 
that  it  is  impossible  to  tell  whether  it  is  a  contemporaneous  or  a  sub- 
sequent deposit.  Hence  metamorphosed  ore  deposits  cannot  be  in- 
cluded under  contemporaneous  deposits,  and  this  leaves  only  the  di- 
visions of  igneous  and  sedimentary  ore  deposits  under  this  head. 

Igneous  Ore  Deposits.  —  In  the  chapters  devoted  to  volcanic  action 
and  igneous  rocks,  it  has  been  shown  that  when  molten  magmas  solidify 
into  rocks  they  generally  undergo  a  process  of  crystallization,  so  that 
the  resultant  mass  consists  of  interlocked  mineral  grains.  All  magmas 
contain  a  little  iron  and  some  of  them  have  small  amounts  of  other 
valuable  ingredients.  By  a  process  called  magmatic  differentiation, 
as  yet  but  little  understood,  these  small  particles  of  valuable  ingredients 
are  sometimes  gathered  together,  during  the  crystallization  of  the  mag- 
ma, into  bodies  of  sufficient  size  and  value  to  constitute  ore  deposits. 
Grains  of  magnetite  may  thus  be  crystallized  out  and  segregated  into 
bodies  of  iron  ore.  Deposits  of  corundum,  chromite,  diamonds,  and 
nickel-bearing  pyrrhotite  and  chalcopyrite  are  also  formed  in  this  man- 
ner; platinum  is  sometimes  associated  with  the  chromite  or  nickel. 
These  deposits  do  not  consist  of  masses  of  pure  ore  minerals;  varying 
amounts  of  rock  minerals  are  always  mixed  with  them.  They  are  sim- 
ply parts  of  the  igneous  rock  where  the  ore  minerals  preponderate  over 


ORE    DEPOSITS  423 

the  rock-making  minerals  and  thus  reverse  the  conditions  usually  ex- 
isting in  igneous  rocks,  where  the  same  ore  minerals  are  scattered 
amongst  the  rock  minerals,  in  minute  specks.  It  must  not  be  supposed 
that  all  magmas  during  their  solidification  give  rise  to  these  deposits; 
if  this  were  the  case  such  deposits  would  be  common  features,  while 
in  reality  they  are  rare. 

Any  one  magma  will  give  rise  to  only  one  kind  of  ore,  such  as  iron 
or  chromite,  but  not  to  both.  Certain  associations  between  the  kind 
of  ore  and  the  composition  of  the  igneous  rock  have  been  observed. 
For  example,  chromite  and  platinum  occur  only  with  peridotite  or  its 
altered  product  serpentine,  and  n  ckel-copper  deposits  occur  only  with 
a  variety  of  gabbro.  Experience  shows  that  the  contemporaneous 
deposits  are  all  restricted  to  intrusive  rocks  and  usually  to  the  deeper- 
seated  types. 

The  igneous  ore  deposits,  of  course,  generally  lie  within  the  igneous 
rock  tself,  because  they  are  a  part  of  it,  and  occur  either  around  the 
margins  of  the  intrusive  or  in  its  center.  This  means  that  the  deposits 
are  restricted  to  a  particular  igneous  rock  and  may  be  found  only  where 
the  igneous  rock  occurs;  the  places  where  these  ores  are  to  be  sought 
are  thus  definitely  indicated.  The  shape  of  the  deposits  is  irregular 
and  their  size  is  variable,  depending  on  the  size  of  the  intrusive.  The 
smaller  the  intrusive,  the  smaller  are  the  deposits,  and  vice  versa. 

Examples  of  this  type  are  to  be  found  in  the  magnetite  deposits  of  the  Adiron- 
dack region  in  New  York  State.  They  occur  as  large  ore  bodies  in  the  center,  or 
about  the  margins,  of  the  intrusives,  and  the  valuable  iron  mineral  is  separated  from 
the  valueless  rock  minerals.  Many  of  the  Adirondack  deposits  contain  titanium  in 
the  form  of  ilmenite,  which,  owing  to  its  infusibility,  makes  the  smelting  and  ex- 
traction of  the  iron  difficult.  In  consequence  of  this  the  deposits  until  recently  were 
of  little  value,  but  now  the  magnetite  and  ilmenite  are  separated  by  electro-magnetic 
methods,  and  the  deposits  are  being  exploited.  There  is  another  type  of  igneous 
magnetite  deposit  in  which  the  separation  between  rock  and  ore  took  place  at  great 
depth  before  the  rock  magma  was  intruded  in  the  places  where  we  now  find  it.  The 
magnetite,  with  its  small  content  of  rock  minerals,  was  intruded  separately,  in  a 
molten  state,  into  the  rocks.  The  valuable  magnetite  deposits  of  Sweden  are  con- 
sidered to  be  of  this  class. 

The  nickel-copper  deposits  of  Norway,  and  of  Sudbury,  Ontario,  are  also  ex- 
amples. In  addition  to  yielding  much  copper,  the  mines  at  Sudbury  are  the  greatest 
producers  of  nickel  in  the  world.  The  ore  consists  of  nickeliferous  pyrrhotite  and 
chalcopyrite,  with  smaller  amounts  of  pyrite  and  pentlandite  mixed  with  gabbro 
(norite)  country  rock.  Appreciable  amounts  of  gold,  silver,  platinum,  and  pal- 
ladium are  also  obtained.  The  deposits  are  large  masses  with  irregular  outlines  and 
lie  on  the  margin  of  the  intrusive 

Sedimentary  Ore  Deposits.  —  As  a  sedimentary  series  is  being 
built  up,  certain  beds  may  be  deposited  whose  component  minerals 


424  TEXT-BOOK  OF  GEOLOGY 

happen  to  be  commercially  valuable  and  sufficiently  abundant  to  be 
workable.  Sedimentary  beds  of  iron  ore  are  thus  formed  in  the  manner 
discussed  on  page  181.  They  may  be  as  extensive  as  the  sedimentary 
series  in  which  they  occur,  but  cannot  be  expected  to  extend  beyond  its 
limits.  Iron  is  the  most  important  metal  found  in  deposits  of  this 
type.  As  sedimentary  beds,  the  iron  deposits  partake  of  the  structure 
of  the  rest  of  the  sedimentary  series  and  may  be  horizontal,  folded,  or 
faulted.  They  are  great,  flat  lenses  which  gradually  thin  out  hori- 
zontally. 

The  Clinton  hematites  are  the  most  important  representatives  in  this  country. 
They  are  hundreds  of  square  miles  in  extent  and  stretch  from  New  York  State  to 
Alabama,  though  they  are  not  thick  enough  to  be  worked  in  all  places.  The  ore 
beds  are  hard,  red  hematite  and  contain  from  30  to  50  per  cent  iron.  Their  lime  and 
silica  content  enables  them  to  be  cheaply  smelted.  They  range  up  to  40  feet  in 
thickness  and  lie  between  layers  of  sandstone  and  shale.  In  some  places  three  or 
four  beds  are  present.  In  the  north  they  are  horizontal  or  gently  inclined  but  in 
the  south  they  are  steeply  folded  and  faulted.  Over  600  million  tons  of  Clinton  iron 
ore  are  estimated  to  be  available,  and  the  beds  support  many  mines  of  which  the  most 
important  are  at  Birmingham,  Alaoama.  The  iron  ores  of  the  Lorraine  field,  France, 
the  most  valuable  in  the  world,  are  also  of  this  type.  Their  reserves  are  estimated 
at  5,000  million  ions  of  iron  ore.  Beds  of  manganese  ore  are  also  formed  by  similar 
processes 

Subsequent  Ore  Deposits 

The  subsequent  deposits  are  the  result  of  ore-forming  processes  as 
distinct  from  rock-forming  processes,  and  include  all  ores  formed  sub- 
sequently to  the  rocks  by  which  they  are  enclosed.  Virtually  all  of 
them  are  now  admitted  to  be  the  result  of  deposition  from  solution 
in  water,  whatever  may  have  been  the  physical  condition  of  the  water. 
Their  form,  size,  and  geological  position  are  therefore  dependent  upon 
the  circulation  of  ore-bearing  solutions.  As  previously  indicated,  the 
processes  involved  in  their  deposition  are  (1)  precipitation  in  rock  open- 
ings, (2)  replacement  of  country  rock,  and  (3)  contact-metamorphic 
action  of  intrusive  igneous  masses.  These  processes  operate  either 
independently  or  in  combination  with  each  other,  and  the  nature  of 
the  dominant  process  in  each  case  places  them  in  the  three  main  groups 
given  in  our  table  of  classification.  These  three  groups  will  now  be 
considered. 

Cavity  Fillings.  —  Many  kinds  of  cavities  exist  in  the  rocks,  and 
any  of  them  may  become  filled  with  ore,  each  giving  rise  to  a  deposit 
of  different  form,  since  the  shape  of  the  cavity  controls  the  form  of 
cavity-filled  ore  deposits.  It  must  be  remembered  that  the  cavity  and 
its  filling  are  two  distinct  things;  the  former  was  made  independently 
of  the  latter  and  in  most  cases  existed  for  a  long  time  before  it  became 


ORE    DEPOSITS 


425 


filled  with  ore.     Of  the  different  classes  of  cavities  in  the  rocks,  fissures 
or  fractures  of  various  kinds  are  the  most  numerous  and  important. 

Fissure  Veins.  —  In  its  simplest  form,  a  fissure  vein  is  a  fissure  in 
the  rock  filled  with  mineral  matter  deposited  from  solution.  See  Fig. 
304.  As  stated  on  page  354,  the  rocks  are  much  shattered  and  occupied 


FIG.  304.  —  Gold-quartz  vein;   the  mining  discloses  the  width  of  the  vein  and 
the  wall-rock  on  either  side.     Cook  Mine,  Central  City,  Colo. 

by  fissures  or  faults,  with  the  result  that  hot  mineralizing  solutions 
have  abundant  opportunity  to  circulate  among  the  rocks  and  deposit 
their  mineral  content.  The  methods  by  which  deposition  may  be 
brought  about  have  already  been  discussed.  The  deposition  takes 
place  as  a  coating  or  crust  on  each  wall,  forming  a  sharp  boundary  with 
the  country  rock,  and  layers  are  added  to  each  side  until  the  open  space 
is  filled.  S  nee  the  chemical  character  of  the  solutions  changes  from 
time  to  time,  the  individual  layers  vary  in  mineral  composition,  as 
illustrated  in  Fig.  305.  This  gives  the  fissure  vein  a  banded  appear- 


426 


TEXT-BOOK  OF  GEOLOGY 


ance,  which  is  called  ribbon  structure  or  trustification:     The  fissure  may 
be  tightly  filled,  or  it  may  happen  that  the  last  layers  do  not  exactly 

meet,  in  which  case  there  remain- 
small  openings  lined  with  projecting 
crystals.  These  openings  are  called 
vugs.  (Fig.  306.) 

The  vein  may  or  may  not  be  of 
commercial  value.  If  it  contains  a 
sufficient  quantity  of  material  of 
workable  grade  it  constitutes  a 
valuable  ore  deposit.  As  previously 
mentioned,  the  ore  minerals  usually 
constitute  only  a  portion  of  the 
filling,  and  the  proportion  between 
the  ore  and  gangue  minerals  is  a 
variable  one  not  only  :n  different 
veins,  but  in  different  parts  of  the 
same  vein.  This  variation  consti- 
tutes one  of  the  uncertainties  of 
mining  operations. 

The  portions  of  a  vein  in  which 
the  ore  minerals  are  more  con- 


FIG.  305.  —  Specimen  from  small  fissure 
vein,  three  inches  wide,  showing  the 
wall-rock  on  either  side  beyond  the 
narrow  dark  line,  and  the  banded 
structure.  One-half  natural  size. 

centrated  are  called  ore  shoots.     A 
vein  usually  contains  one  or  more 


b  b      b  b 

FIG.  306.  —  Section  of  a  fissure  vein: 
aa,  wall-rock;  66,  layers  of  ore;  cc, 
gangue;  d,  open  cavity,  or  vng,  lined 


of  thefti,  and  they  are   separated  with  crystals.    Often  the  fissure  filling 

from  each  other  by  leaner  parts  is  separated  from  the  wall-rock  by  a 

or    by    barren    vein  matter.      In  tnin  ]ayer  of  crushed  or  decomposed 

to    the    actual    filling  rock  known  as  selvage>  or>  more  com" 


addition 


the   walls   of   most   fissure  veins     monlv' 

are   altered    and    often   impregnated   with    ore   minerals. 

Since  veins  are  filled  fissures  or  faults,  the  terms  given  for  faults 
in  Chapter  XIV  are  also  applied  to  fissure  veins.     The  position  of  a 


ORE    DEPOSITS  427 

vein  is  definitely  designated  by  giving  its  strike  and  dip.  For  example, 
a  given  vein  may  be  said  to  have  a  strike  of  N.  45°  E.  and  a  dip  of  60° 
N.W.  In  the  case  of  inclined  veins,  and  most  veins  are  inclined,  the 
upper  wall  is  called  the  hanging-wall,  and  the  lower,  the  foot-wall. 
Veins  seldom  occur  singly;  usually  there  are  a  group  of  them  and  their 
strikes  may  be  either  parallel  or  intersecting.  They  may  also  intersect 
each  other  along  their  dip. 

The  length  and  depth  of  veins  can  be  no  greater  than  that  of  the 
fissures  they  fill :  they  may  extend  a  few  hundred  feet  or  a  few  thousand 
feet  along  their  strike,  and  some  have  been  followed  down  their  dip  for 
several  thousands  of  feet.  The  majority  of  veins,  however,  are  rela- 
tively shallow  and  die  out  within  3000  feet  of  the  surface.  In  thickness 
they  vary  from  a  few  inches  (Fig.  305)  to  many  yards,  but  veins  less 
than  10  feet  in  width  are  more  numerous  than  those  over  10  feet.  All 
veins  terminate  both  along  their  strike  and  along  their  dip,  usually  by 
a  gradual  pinching  out  or  by  abutting  against  other  fissures. 

Fissure  veins  are  one  of  the  most  important  class  of  ore  deposits, 
being  the  chief  sources  of  gold,  silver,  copper,  and  other  metals.  Quartz 
is  almost  invariably  the  most  important  gangue  mineral,  so  much  so 
that  fissure  veins  are  frequently  called  "  quartz  veins."  Depending 
on  whether  the  veins  were  formed  at  great  depth,  at  intermediate  depth, 
or  at  shallow  depth,  they  will  exhibit  the  characteristics  and  mineral 
composition  of  the  respective  zones  already  referred  to  on  page  419. 

Fissure  veins  are  not  to  be  thought  of  as  contained  within  two  parallel  planes, 
uniformly  distant  throughout  their  extent,  like  the  covers  of  a  book.  Examples 
that  even  approach  such  simplicity  are  rare.  On  the  contrary,  their  irregularities 
are  numerous;  they  may  widen,  giving  rise  to  chambers  of  ore,  or  they  may  pinch 
until  only  thin  films  or  stringers  are  left  to  show  their  continuation;  after  pinching 
they  may  widen  again,  or  finally  end.  They  twist  or  turn  along  their  strike  or  dip; 
they  branch,  and  perhaps  reunite;  they  send  out  stringers  into  the  walls,  and  are 
cut  by  other  veins.  Again,  masses  of  country  rock,  known  as  horses,  may  be  en- 
closed in  the  vein,  or  a  major  vein  may  be  composed  of  a  number  of  closely  spaced, 
parallel  minor  veins;  finally,  the  vein  may  represent  merely  a  shattered  zone  of  rock 
whose  interstices  are  filled  by  interlacing  films  of  ore.  Veins  are  frequently  cut  and 
displaced  by  faults,  a  feature  that  forms  one  of  the  most  serious  difficulties  in  mining 
operations.  The  discussion  of  faults  in  Chapter  XIV  may  be  applied  here  with 
profit.  A  vein,  in  passing  from  one  rock  to  another,  usually  suffers  some  change; 
it  may  break  up  into  a  number  of  fraying  branches,  or  become  a  wide,  shattered  zone; 
it  may  undergo  a  restriction  in  width,  or  abruptly  terminate. 

Examples  of  fissure  veins  are  world-wide,  and  many  occur  throughout  the  western 
United  States.  Typical  gold  veins  are  to  be  found  in  California;  they  are  persistent 
fissures  enclosed  within  a  country  rock  of  slate,  and  consist  of  massive  white  quartz 
through  which  is  scattered  more  or  less  pyrite.  The  gold  forms  only  a  minute  part 
of  the  ore,  and  when  it  can  be  seen  it  is  in  the  form  of  small  specks  of  native  metal; 
usually  it  is  concealed  in  the  pyrite.  Typical  copper  veins  occur  at  Butte,  Montana, 


428 


TEXT-BOOK  OF   GEOLOGY 


where  over  two  thousand  miles  of  underground  workings  expose  the  veins  to  a  depth 
of  3600  feet.  They  all  occur  in  a  large  granite  intrusive  and  intersect  and  fault  each 
other  in  strike  and  dip.  The  filling  consists  of  pyrite  and  numerous  ore  minerals 
of  copper,  silver,  and  zinc.  Quartz  is  the  chief  gangue  mineral.  The  wall-rocks  are 
also  impregnated  with  ore  minerals.  Many  other  instances  of  fissure  veins  contain- 
ing other  metals  might  be  cited,  but  the  above  will  serve  as  examples. 

Other  Rock  Openings.  —  Although  fissures  are  the  most  numerous 
and  important  form  of  filled  cavities,  there  are  many  other  kinds  of 
openings  in  the  rocks  which  have  been  similarly  filled  by  ore.  In  the 
famous  copper-mining  district  of  the  Lake  Superior  region,  the  "  blow 
holes  "  of  vesicular  lavas  (page  207)  have  been  filled  by  native  copper 
and  other  minerals,  and  immense  low-grade  copper  deposits  occur  where 
the  filled  cavities  are  sufficiently  numerous  and  spaced  closely  together. 
Pore  spaces  of  rocks  have  also  been  filled  with  small  specks  of  ore 
mineral  and  have  given  rise  to  disseminated  ore  deposits  of  large  volume 
but  low  grade.  Caves,  formed  by  solution  in  soluble  rocks  (page  162), 
and  joint  cracks  and  openings  between  sedimentary  strata,  called 
pitches  and  flats,  afford  opportunity  for  deposition  of  mineral  matter 
from  circulating  solutions;  and  throughout  the  Mississippi  Valley, 
particularly  in  Wisconsin,  such  cavities  have  been  filled  by  valuable 
deposits  of  zinc  and  lead.  See  Fig.  307.  Another  type  of  rock-opening, 


FIG.  307.  —  Cavity-filled  deposit  shows  an  open  cavity  coated  by  deposited 
layers  of  gangue  and  ore  mineral,  the  latter  black.  The  stalactites  and  stalagmites 
of  the  original  opening  show  that  the  cavity  existed  before  the  deposition.  Masses 
of  ore  mineral  fell  from  the  roof  before  the  last  deposit  was  made. 

formed  at  the  crests  of  anticlines  by  the  close  folding  of  stratified  rocks, 
and  resembling  the  openings  produced  when  one  squeezes  together 
a  pile  of  papers,  has  resulted  in  valuable  gold  deposits  at  Bendigo, 
Australia,  and  at  Nova  Scotia,  Canada.  Because  of  their  resemblance, 
in  vertical  section,  to  a  saddle,  they  are  termed  saddlereefs. 

Replacement  Deposits.  —  Replacement  deposits  are  formed  as  a 
result  of  the  gradual  chemical  substitution  of  ore  for  country  rock,  by 
the  action  of  circulating  mineralizing  solutions.  They  are  not  the 


ORE    DEPOSITS 


429 


result  of  the  filling  of  pre-existing  cavities;  the  spaces  now  occupied  by 
ore  were  formed  and  filled  at  the  same  time,  and  the  interchange  of 
ore  for  country  rock  took  place  molecule  by  molecule,  or  particle  by 
particle,  un'il  a  mass  of  ore  occupied  the  space  of  the  mass  of  rock. 
The  process  may  be  roughly  illustrated  by  comparing  it  to  that  taking 
place  in  a  brick  wall  in  which  a  silver  brick  is  substituted  for  a  clay 
brick  and  the  action  repeated  until  eventually  all  the  clay  bricks  have 
been  replaced  by  silver  bricks,  and  the  resulting  silver  wall  has  the 
same  volume  and  structure  as  the  original  wall.  The  illustration  will 
portray  the  actual  conditions  more  closely  if  we  consider  that  here  and 
there  some  of  the  clay  bricks  escaped  substitution,  because,  in  nature, 
unreplaced  fragments  of  country  rock  are  frequently  left  surrounded 
by  ore.  These  unsupported  residual  nuclei,  as  they  are  called,  afford 
convincing  proof  of  replacement,  because  by  no  other  process  could  they 
be  left  suspended  in  ore.  See  Fig.  308.  They  enable  replacement  de- 


FIG.  308.  —  Replacement  ore  deposit.  The  dark  area,  ore.  The  suspended  blocks 
of  unaltered  limestone  in  the  ore  show  that  there  could  not  have  been  an  open 
cavity  when  the  ore  was  deposited. 

posits  to  be  distinguished  from  cavity  fillings,  because  if  the  cavity  was 
there  first,  such  fragments  could  not  remain  suspended  in  mid-air,  but 
obviously  would  rest  on  the  bottom  of  the  opening. 

The  action  of  substitution  starts  at  a  number  of  centers,  each  of 
which  gradually  enlarges  until  they  coalesce  and  the  whole  mass  of 
rock  is  more  or  less  completely  replaced,  giving  rise  to  massive  replace- 
ment deposits.  The  process,  however,  may  be  interrupted  before  the 
centers  coalesce,  in  which  case  disseminated  replacement  deposits  result. 
In  deposits  of  the  latter  class,  the  ore  forms  but  a  small  part  of  the 
deposit,  the  remainder  being  country  rock,  while  in  those  of  the  former 
class,  the  volume  of  ore  is  much  greater  and  may  even  constitute  the 
whole  deposit.  A  cavity  or  opening  of  some  sort  is  necessary  for  the 
operation  of  replacement,  in  order  to  give  the  ore-forming  solutions 
access  to  the  rocks,  but  only  a  very  small  opening  is  needed.  Fissures 
are  the  most  common  of  such  channels  of  access,  and  the  walls  of  the 
fissure  may  be  replaced  by  ore,  giving  rise  to  replacement  veins.  These 


430  TEXT-BOOK  OF  GEOLOGY 

resemble  fissure  veins  in  their  shape  and  dimensions,  but  their  width  is 
usually  more  irregular. 

Replacement  ore  deposits  may  be  formed  in  any  variety  of  rock  that 
is  susceptible  to  attack  by  the  circulating  solutions.  Limestones  are 
the  most  frequent  hosts  for  such  ore  deposits,  since  they  are  com- 
paratively soluble,  but  many  large  ore  bodies  occur  in  the  other  rocks. 
The  shape  of  replacement  deposits  is  usually  very  irregular,  particularly 
in  the  case  of  the  massive  and  disseminated  replacements;  their  out- 
lines are  determined  by  the  points  where  the  replacing  action  happened 
to  cease.  Some  replacement  deposits  occur  in  thin  beds  of  soluble 
limestone  enclosed  between  layers  of  insoluble  strata;  in  this  case  the 
limestone  may  be  completely  replaced  over  its  total  thickness,  so  that 
the  resulting  ore  deposit  has  sharp,  regular  boundaries  above  and 
below.  The  replacement  veins  have  the  most  regular  shape  of  the 
different  replacement  deposits,  being  tabular  in  form. 

Replacement  deposits  may  attain  great  size,  and  many  of  the 
world's  largest  bodies  of  ore  belong  to  this  class.  These  deposits  are 
worked  for  many  metals.  Vast  quantities  of  copper,  lead,  and  zinc, 
and  considerable  gold  and  silver,  are  obtained  rom  them.  The  relative 
proportion  of  ore  minerals  to  gangue  minerals  is  about  the  same  as  in 
other  types  of  deposits. 

As  was  previously  pointed  out,  some  form  of  cavity  is  necessary  to  allow  access 
of  the  mineralizing  solutions  to  the  rocks.  With  small  cavities  the  amount  of  ore 
produced  by  cavity-filling  is  negligible,  but  with  larger  cavities  there  may  be  both 
cavity-filling  and  replacement.  In  fact  most  cavity-filling  is  accompanied  by  more 
or  less  replacement  of  the  rock  surrounding  the  cavities.  It  is  thus  evident  that 
between  filled  cavities  and  replacement  deposits  all  degrees  of  gradation  must  exist. 
Yet  at  either  end  of  this  line  of  gradation  each  type  stands  forth  clearly,  and  definite 
examples  of  each  can  be  readily  recognized  by  the  careful  observer.  Where  the 
predominant  process  is  cavity-filling  we  call  the  resulting  ore-bodies  cavity-filled 
deposits,  and  where  replacement  has  been  the  dominant  factor  in  this  formation,  they 
are  called  replacement  deposits.  It  is  essential,  however,  not  only  as  a  matter  of 
convenience,  but  of  scientific  value,  that  the  distinct  types  should  be  recognized. 

Examples  of  replacement  deposits  are  numerous  throughout  Colorado,  Utah, 
Montana,  Idaho,  and  Arizona.  The  great  lead-silver-zinc  deposits  of  Leadville, 
Colorado,  belong  to  this  type.  Large,  irregular  ore  bodies  occur  in  limestone,  in 
many  places  replacing  the  whole  bed  of  limestone  between  porphyry  walls.  The 
ore  consists  chiefly  of  pyrite,  zincblende,  silver-bearing  galena,  and  their  oxidation 
products,  mixed  with  quartz  and  other  gangue  minerals.  In  the  Coeur  d'Alene  dis- 
trict of  Idaho,  great  replacement  deposits  of  lead  and  silver  have  been  mined  for 
years,  and  produce  about  one-third  of  the  lead  output  of  the  United  States.  Prac- 
tically all  of  the  metals  are  obtained  from  silver-bearing  galena  which  is  disseminated 
in  small  amounts  through  shattered  quartzite.  A  little  pyrite,  chalcopyrite,  and 
zincblende  occur,  and  these  yield  lesser  amounts  of  gold,  copper,  and  zinc.  Siderite 
and  quartz  are  the  chief  gangue  minerals.  Each  ton  of  ore,  as  mined,  contains  about 


ORE    DEPOSITS  431 

8  per  cent  of  lead  (160  Ibs.  per  ton)  and  6  ounces  of  silver.  An  example  of  replace- 
ment copper  deposits  is  to  be  found  at  Bisbee,  Arizona,  where  large  ore  bodies  con- 
sisting chiefly  of  pyrite,  chalcopyrite,  bornite,  chalcocite,  copper  carbonate,  quartz, 
and  altered  country  rock,  occur  in  limestone  and  porphyry.  Copper  is  the  chief 
mineral  sought  and  the  copper  content  of  the  ore  ranges  from  2  to  14  per  cent. 

Contact  Metamorphic  Deposits.  —  It  has  already  been  stated  in 
Chapter  XII  (and  on  page  350)  that  when  a  mass  of  molten  magma 
is  intruded  into  sediments,  and  solidifies  to  igneous  rock,  certain  effects 
are  produced  in  the  enclosing  rocks  in  the  vie  nity  of  the  contact  which 
are  included  under  the  term  contact  metamorphism.  The  main  effect 
is  the  hardening,  baking,  and  recrystallization  of  the  beds,  with  pro- 
duction of  new  minerals,  and  is  caused  by  the  heat  and  the  vapors  of 
various  kinds  that  escape  from  the  cooling  magma  into  the  adjacent 
formations.  The  intrusion  of  the  magma  may  have  shattered  the  beds 
and  the  recrystallizafcion  may  have  resulted  in  shrinkage,  giving  rise 
to  small  openings.  Such  cavities  would  permit  the  introduction  of 
metalliferous  vapors  given  off  from  the  magma,  and  contact-metamorphic 
ore  deposits  might  be  formed.  See  Fig.  309.  The  result  is  a  distinctive 


FIG.  309.  —  Diagram  showing  a  cross  section  of  contact-metamorphic  deposits. 
Stippled  area  represents  contact-metamorphic  zone  and  black  ore. 

group  of  gangue  and  ore  minerals  in  an  association  with  one  another 
which  is  readily  recognizable  and  makes  the  contact-metamorphic  origin 
of  the  deposits  a  matter  of  simple  and  easy  determination. 

Certain  conditions  are  necessary  for  the  formation  of  this  type  of 
deposit.  The  igneous  intrusion  must  be  a  deep-seated  one  and  there- 
fore great  erosion  is  necessary  to  expose  it  at  the  surface.  The  igneous 
rock  is  usually  of  a  feldspathic  nature,  such  as  granite  or  diorite,  and 
valuable  deposits  occur  only  where  the  rocks  which  are  intruded  are 
limestones  or  limy  shales.  It  does  not  follow,  however,  that  all 
intrusive  igneous  rocks  exert  contact  metamorphism  nor  that  contact 
metamorphism  is  always  accompanied  by  ore  deposition.  Intrusion 


432  TEXT-BOOK  OF  GEOLOGY 

may  take  place  with  or  without  contact  metamorphism  of  the  surround- 
ing rocks,  and  ore  deposition  is  more  often  absent  from  such  contact 
metamorphism  than  present. 

In  this  type  of  deposit  there  is  usually  more  than  one  body  of  ore. 
The  individual  ore  bodies  are  separated  from  each  other  by  variable 
intervals  of  metamorphosed  rock  and  are  irregularly  distributed  through 
the  metamorphic  zone  surrounding  the  igneous  intrusion  (page  351) 
and  usually  adjacent  to  the  contact.  They  are  bunchy  in  character 
and  vary  greatly  in  shape  and  size.  See  Fig.  309.  The  deposits  are 
characterized  by  the  development  of  numerous  and  unusual  gangue 
minerals,  such  as  garnet  and  amphibole;  pyrrhotite,  pyrite,  arsenopyrite, 
and  chalcopyrite,  and  other  sulphides  of  the  metals  are  usually  present. 
Probably  the  most  characteristic  feature  of  contact-metamorphic  de- 
posits, and  one  by  which  they  can  be  readily  recognized,  is  the  intimate 
intergrowth  of  the  silicate  minerals  with  magnetite  or  hematite  and 
sulphides.  The  ore  minerals  are  usually  scattered  in  small  particles 
through  the  gangue  minerals  or  altered  rock,  and  constitute  a  minor 
part  of  the  ore.  These  deposits  have  been  worked  for  iron,  copper, 
lead  and  zinc,  and  gold.  A  little  silver  usually  accompanies  the  copper 
and  gold  group.  These  contact-metamorphic  ore  deposits  are  not  as 
numerous  or  as  important  as  the  other  types,  though  individual  de- 
posits may  be  of  great  value.  Usually  these  bodies  are  low-grade,  and 
their  irregular  distribution,  shape  and  size,  make  them  difficult  and 
hazardous  to  mine. 

Examples  of  contact-metamorphic  deposits  are  to  be  found  in  regions  where 
extensive  erosion  has  revealed  the  larger  intrusions  of  deep-seated  origin,  notably  in 
the  mountainous  regions  of  the  eastern  and  western  United  States.  Copper  de- 
posits of  this  type  have  been  mined  at  Clifton-Morenci,  Arizona,  where  chalcopyrite, 
admixed  with  magnetite,  pyrite,  zincblende,  garnet,  and  other  minerals,  occurs  in 
altered  limestones,  adjacent  to  a  stock  of  granite  porphyry.  Gold  deposits  in  which 
the  gold  occurs  in  arsenopyrite,  mixed  with  chalcopyrite,  zincblende,  and  silicate 
minerals,  occur  in  limestone  at  Hedley,  British  Columbia.  The  deposits  have  already 
yielded  several  million  dollars  from  ore  which  is  worth  from  $6  to  $14  per  ton.  Ex- 
tensive magnetite  deposits  of  this  type  occur  at  Cornwall,  Pennsylvania. 

Disintegration  or  Secondary  Deposits 

As  previously  mentioned,  disintegration  deposits  are  those  formed  as 
a  result  of  mechanical  or  chemical  decay  of  metalliferous  rock  masses, 
and  later  concentration  at  the  surface.  The  operation  of  the  me- 
chanical and  chemical  agents  has  already  been  explained  in  this  volume. 
The  activities  of  these  agents  have  been  discussed  in  connection  with 
the  weathering  of  rocks,  the  formation  of  soil,  and  erosion,  and  the 


ORE    DEPOSITS  433 

results  have  been  pointed  out  in  the  descriptions  of  the  chemical  work 
of  underground  water  and  the  formation  of  stratified  rocks. 

In  the  same  way  that  the  rocks  are  subject  to  these  processes,  so 
also  are  the  ore  minerals  contained  in  them.  Some  tend  to  be  broken 
up  and  to  be  transported  elsewhere,  others  to  be  taken  into  solution 
and  carried  away,  and  some  to  remain  behind  while  the  materials  sur- 
rounding them  are  carried  away  in  solution.  Different  kinds  of  second- 
ary deposits  result  from  these  three  processes. 

Mechanical  Concentrations :  Placers.  —  In  this  type  of  deposit 
nature  has  operated  to  produce  the  results  achieved  by  man  when  he 
mines,  crushes,  and  concentrates  ore  to  obtain  the  valuable  ore  minerals. 
Weathering  and  erosion  break  up  the  bed-rocks  with  their  contained 
ore  minerals,  which  may  have  originally  occurred  in  regular  deposits  of 
ore,  or  as  worthless,  sparsely  scattered  grains.  The  particles  of  ore 
minerals  are  thus  separated  from  the  surrounding  gangue  or  country 
rock,  and  the  disintegrated  materials,  ore  minerals  and  rock  particles 
alike,  are  moved  slowly  down  the  surface  slopes  to  the  nearest  stream. 
There  the  water  sweeps  away  the  lighter  rock  particles,  and  the  heavier 
ore  minerals  sink  to  the  bottom  or  are  moved  relatively  short  distances. 
(Page  44.)  As  may  thousands  of  tons  of  debris  are  thus  moved  to  the 
streams,  the  few  ore  minerals  in  each  ton  of  debris  will  be  gradually 
concentrated  in  the  gravels  of  the  stream  bottom  until  there  is  accumu- 
lated a  deposit  of  sufficient  size  to  be  workable.  An  accumulation  of 
this  kind  is  commonly  called  a  placer  deposit,  and  the  operation  of  ex- 
tracting the  valuable  minerals  is  called  placer  mining,  in  contrast  to 
lode  mining  from  bed-rock  deposits. 

Certain  conditions  are  necessary  to  produce  placer  deposits.  The 
ore  minerals  must  be  of  such  a  nature  that  they  are  insoluble  in  surface 
waters,  else  they  will  be  taken  into  solution.  Also,  they  must  be  heavier 
than  the  rock  particles  of  the  same  size,  so  that  they  will  sink  in  moving 
water  while  the  rock  particles  are  swept  away;  and  the  streams  must 
have  sufficient  velocity  to  be  able  to  move  the  rock  particles.  If  the 
stream  velocity  is  great,  the. ore  particles  may  also  be  carried  along  until 
a  olace  is  reached  where  the  current  slackens;  there  they  will  be  dropped. 
For  example,  a  gold-quartz  vein,  containing  specks  of  native  gold  to 
the  amount  of  only  one-tenth  of  an  ounce  for  each  ton  of  ore,  may  be 
disintegrated  and  eroded.  The  gold  particles,  being  insoluble  and 
heavy,  will  accumulate  in  the  gravels  at  the  bottom  of  the  stream  while 
most  of  the  quartz  is  swept  away.  For  every  ton  of  ore  eroded,  a  part 
of  the  one-tenth  ounce  of  gold  will  accumulate,  until  perhaps  several 
thousands  of  ounces  of  gold  (worth  $17  to  $20  per  ounce)  are  scattered 
over  the  gravelly  bottom  of  the  stream.  In  this  way  spectacular  de- 


434  TEXT-BOOK  OF  GEOLOGY 

posits  of  placer  gold  have  been  formed,  and  many  millions  of  dollars 
have  been  extracted  from  them.  The  number  of  valuable  minerals 
that  meet  the  requirements  stipulated  above  are  relatively  few,  since, 
as  will  be  seen  later,  most  of  them  are  chemically  attacked  by  surface 
waters,  or  are  of  low  specific  gravity.  Gold  is  by  far  the  most  common 
mineral  so  accumulated;  but  platinum,  tinstone  (cassiterite,  SnO3), 
quicksilver,  and  gem-stones  also  occur  in  placer  deposits. 

The  desired  mineral  in  placer  deposits  is  loosely  scattered  through  gravels  and 
can  be  readily  obtained  by  simple  methods  of  washing  the  heavier  substances  from 
the  gravel.  The  gold  (or  other  valuable  substance)  does  not  occur  as  an  accumulation 
of  the  pure  metal,  but  is  always  mixed  with  a  greater  amount  of  gravel.  Occasion- 
ally pockets  or  streaks,  often  called  bonanzas,  have  been  found,  in  which  a  shovelful 
of  gravel  may  contain  a  hundred  dollars  in  gold,  or  more.  Usually,  however,  it  is 
scattered  in  much  smaller  amounts.  Most  of  the  gold  is  in  the  form  of  fine  specks 
called  "dust,"  but  to  the  joy  of  tho  miner  larger  lumps  or  nuggets  frequently  occur, 
some  of  which  may  have  a  value  of  several  thousand  dollars. 


FIG.  310.  —  Gold  placer  mining  in  operation.  The  sand  and  gravel  containing 
the  fine  gold  particles  is  being  washed  down  by  powerful  jets  of  water  into  sluice- 
boxes  where  the  gold  is  caught  by  riffles.  Cariboo  District,  British  Columbia, 
Canada. 

Undoubtedly  the  earliest  primitive  mining  was  from  deposits  of  this  type. 
The  ease  of  extraction  and  the  occasional  richness  of  the  deposits  make  them  eagerly 
sought.  It  was  such  gold  deposits  that  gave  rise  to  the  great  California  gold  rush  in 
49,  to  the  Klondyke  "stampede"  in  Yukon,  Canada,  and  to  the  rich  discoveries 
in  Australia,  Alaska,  and  other  places.  The  hardy  miner  requires  only  a  shovel  and 
pan  to  extract  the  gold;  the  gravel  and  water  are  placed  in  a  small  pan,  and  by  dex- 


ORE    DEPOSITS  435 

terous  rotation  by  hand  the  gravel  may  be  washed  free  from  the  gold.  This  method 
is  suitable  for  the  richer  gravels  only;  the  process  of  extracting  gold  from  lower- 
grade  gravels  consists  in  shovelling  the  gravel,  or  sluicing  it  by  means  of  water  jets 
(see  Fig.  310),  into  long,  narrow  troughs,  called  sluice-boxes,  which  have  crossbars, 
or  riffles,  at  the  bottom.  The  sand  and  gravel  are  washed  along  the  boxes  by  a 
current  of  water,  and  the  gold,  because  of  its  higher  specific  gravity,  sinks  and  is 
caught  against  the  riffles.  Still  more  refined  methods  utilize  large  mechanically 
operated  dredges  which  can  handle  rapidly  great  volumes  of  gravel,  and  material 
containing  gold  particles  to  the  value  of  only  8  cents  in  every  cubic  yard  of  gravel 
may  be  operated  profitably.  A  considerable  proportion  of  the  gold  of  the  world  has 
been  won  from  deposits  of  this  type,  as  well  as  practically  all  of  our  platinum,  of 
which  the  chief  source  is  the  Ural  Mountains,  Russia.  Placer  tin  deposits  are  ex- 
tensively mined  in  the  East  Indies. 

Although  most  of  the  concentration  of  placer  deposits  has  taken  place  by  means 
of  moving  water,  wind  has  produced  similar  results  in  Australia. 

Chemical  Concentrations.  —  In  the  process  of  weathering  and 
erosion,  some  minerals  of  disintegrating  ore  deposits  may  be  taken  into 
solution  by  surface  waters,  and  be  redeposited  elsewhere.  For  ex- 
ample, iron  may  be  dissolved  and  transported  a  short  distance  and  re- 
deposited  on  the  surface  to  give  rise  to  limonite  deposits.  Other  sub- 
stances may  also  be  dissolved  and  carried  down  into  the  original  ore 
deposits  before  being  precipitated,  but  as  this  involves  the  important 
subject  of  surface  alteration  of  ore  deposits,  it  will  be  considered  separ- 
ately in  a  succeeding  section 

Residual  Concentrations.  —  Insoluble  ore  minerals  may  occur  scat- 
tered through  a  soluble  rock  such  as  limestone,  and,  in  the  process  of 
weathering,  the  limestone  may  be  dissolved  and  carried  away,  leaving 
the  insoluble  particles  to  accumulate  behind.  The  process,  if  continued 
long  enough,  may  give  rise  to  bodies  of  ore,  which  are  termed  residual 
deposits.  Deposits  of  manganese  and  iron  have  been  formed  in  this 
manner. 

Surface  Alteration  and  Enrichment  of  Ore  Deposits 

Most  bed-rock  deposits  were  originally  formed  at  depth  and  have 
been  exposed  at  the  surface  by  the  gradual  erosion  of  the  overlying  rocks. 
They  are  therefore  affected,  like  the  rocks  that  enclose  them,  by  the 
agencies  of  weathering,  and  by  surface  waters  with  their  contained  gases. 
As  has  been  shown  in  previous  chapters,  the  agencies  of  weathering  and 
decay  operate  above  the  ground-water  level,  and  because  the  minerals 
in  this  belt  become  oxidized,  it  is  referred  to  as  the  zone  of  oxidation. 
These  agencies  produce  profound  changes  in  the  upper  parts  of  ore 
deposits  to  which  such  surface  waters  have  access;  certain  minerals 
are  oxidized  and  yield  solvents  which  dissolve  the  other  minerals  present, 
and  thus  leach  the  upper  part  of  an  ore  deposit.  As  these  cold  dilute 


435A 


TEXT-BOOK  OF  GEOLOGY 


solutions  slowly  trickle  downward  through  the  deposit,  part  of  their 
metallic  content  may  be  precipitated  in  the  zone  of  oxidation,  or  they 
may  continue  down  to  the  water  level  where  the  remainder  of  their 
dissolved  metals  are  precipitated  as  secondary  sulphides  of  the  metals. 
Below  this  point  the  ore  is  not  affected,  and  it  remains  in  its  original 
condition.  As  a  result  of  these  processes  an  ore  deposit  develops  three 
pronounced  zones:  (1)  An  upper  oxidized  zone;  (2)  a  zone  where  the 
metals  leached  out  of  the  oxidized  zone  have  been  reprecipitated  as 
secondary  sulphides  enriching  those  previously  present  in  the  ore,  called 
the  zone  of  secondary  enrichment;  and  (3)  the  lower  unchanged  part  of 
the  deposit  called  the  primary  zone.  See  Fig.  311.  This  zonal  ar- 

d 


FIG.  311.  —  Diagram  illustrating  alteration  of  an  ore-vein  and  a  secondary 
enrichment  zone:  aa,  country  rock;  66,  level  of  ground  water;  cd,  ore-vein;  d, 
capping  of  gossan;  e,  leached  and  barren  part  of  vein;  /,  concentrated  oxidized  ore 
with  carbonates;  g,  secondary  enrichment  zone  of  highly  metalled  sulphides  and 
metals;  h,  normal  part  of  vein  with  unchanged,  lower-metalled  sulphides. 

rangement  is  one  of  the  chief  means  by  which  the  effects  of  surface 
alteration  and  enrichment  may  be  recognized.  The  processes  which 
operate,  and  the  results  achieved  in  the  different  zones  can,  perhaps, 
be  best  illustrated  by  means  of  one  simple  ore  deposit,  such  as  the 
copper  vein  shown  in  Fig.  311,  consisting  of  quartz,  pyrite,  and  chal- 
copyrite  exposed  at  the  surface  in  its  original  condition. 

Solution  of  Metals  in  Oxidized  Zone.  —  In  the  same  way  that  a 
piece  of  iron  exposed  to  oxygen  and  water  becomes  rusty,  so  does  the 
pyrite  of  the  ore  deposit.  It  is  chemically  destroyed,  and  ferric  sul- 
phate, sulphuric  acid,  and  limonite,  all  products  of  oxidation,  are  formed. 


ORE    DEPOSITS  435B 

The  copper  in  the  chalcopyrite  is  readily  dissolved  by  the  ferric  sul- 
phate, goes  into  the  solution  in  the  form  of  copper  sulphate,  and  slowly 
trickles  down  through  the  deposit.  If  other  minerals  are  present  in 
the  deposit  they  may  also  be  similarly  dissolved.  The  limonite,  how- 
ever, does  not  go  into  solution  but  stays  behind  and  stains  the  quartz 
a  rusty  color.  The  spaces  originally  occupied  by  the  sulphides  are 
now  cavities,  and  the  outcrop  of  the  vein  is  a  rusty  cavernous  mass  of 
quartz,  barren  of  ore  minerals,  and  is  known  as  gossan  or  "  iron  hat." 
Gossans  may  also  be  formed  in  ore  deposits  consisting  only  of  valueless 
pyrite.  For  these  reasons  the  value  and  mineral  character  of  many 
veins  discovered  on  the  surface  cannot  be  determined  until,  at  great 
expense,  they  are  penetrated  at  depth. 

The  process  of  oxidation  and  solution  will  extend  downward  as  far 
as  oxygen  is  available,  usually  to  the  water  level,  and  the  depth  may 
reach  several  hundred  feet.  Because  of  this,  valuable  deposits  have 
remained  undiscovered  for  long  periods  of  time. 

Similarly,  silver,  zinc,  and  other  metals  are  removed  from  the  oxidized  zone. 
Gold  is  sometimes  removed  in  the  same  manner,  but  more  often  it  resists  solution 
and  remains  behind  in  the  rusty  quartz,  in  the  metallic  state,  forming  what  is  known 
as  " free-milling"  ore,  so  named  because  of  the  ease  with  which  the  gold  can  be 
extracted  directly  by  quicksilver,  and  in  contrast  to  its  usual  occurrence  in  sulphides 
from  which  it  can  be  removed  only  by  special  processes.  In  many  cases,  the  "free 
milling"  gold  ore  has  undergone  a  residual  enrichment.  For  example,  if  the  original 
deposit  contained  $5  in  gold  per  ton  of  ore,  and  half  of  a  given  ton  of  ore  is  removed 
by  solution  and  the  gold  remains  behind,  there  is  then  $5  in  half  a  ton,  or  $10  in  a  whole 
ton,  so  that  the  ore  of  the  oxidized  zone  has  been  enriched  over  that  of  the  original 
ore.  Under  such  conditions,  contrary  to  the  usual  conception,  the  ore  will  become 
leaner  in  depth  as  the  bottom  of  the  oxidized  zone  is  passed. 

In  regions  of  heavy  glaciation  the  oxidized  zones  have  been  eroded  and  the 
primary  ores  exposed  at  the  surface.  The  retreat  of  the  ice  has  been  too  recent  to 
allow  any  appreciable  oxidation  and  solution  since  that  time. 

Precipitation  of  Minerals  in  Oxidized  Zone.  —  The  copper  sulphate 
solution,  formed  in  the  zone  of  oxidation  in  its  journey  down  through 
the  deposit,  may  lose  all  or  a  part  of  its  copper  content  before  it  reaches 
the  water  level.  It  may  be  evaporated,  in  which  case  copper  sulphate 
minerals  are  deposited  in  the  oxidized  zone.  Should  it  come  in  contact 
with  soluble  silica,  the  copper  and  the  silica  will  combine  to  form  the  blue 
copper  silicate,  chrysocolla  (CuSi03  +  2H2O).  Or,  if  the  copper  sul- 
phate solution  meets  limestone,  it  will  react  with  the  calcium  carbonate 
to  form  the  blue  and  green  carbonates  of  copper,  namely  azurite 
(2CuCO3Cu(OH)2),  and  malachite,  (CuCO3Cu(OH)2).  Again,  it  may 
mingle  with  other  solutions,  and  native  copper  or  cuprite  (Cu2O)  will 
be  formed.  All  these  minerals  will  be  deposited  in  the  oxidized  zone, 
and  usually  near  the  bottom  of  it. 


435c  TEXT-BOOK  OF  GEOLOGY 

Similarly,  if  the  deposit  happens  to  contain  zinc,  the  carbonate  and  silicate  of 
zinc  —  smithsonite  (ZnCO3)  and  calamine,  (H2Zn2SiO5)  —  may  be  deposited;  also  the 
lead  carbonate,  cerussite  (PbCO3),  and  the  lead  sulphate,  anglesite  (PbSO4)  may 
likewise  be  formed.  If  silver  be  present  it  may  be  redeposited  as  native  silver  or  as 
silver  chloride  (AgCl).  Carbonates  are  always  to  be  expected  where  a  deposit  en- 
closed in  limestone  has  been  oxidized.  All  of  the  ore  minerals  mentioned,  with  the 
exception  of  native  copper  and  native  silver,  which  may  sometimes  occur  as  primary 
minerals,  are  invariably  formed  as  a  result  of  these  secondary  processes  of  oxidation. 
The  possibility  of  their  constituting  large  ore  bodies  must  always  be  viewed  with 
suspicion,  because  they  will  disappear  at  the  bottom  of  the  zone  of  oxidation  and  give 
place  to  ore  of  entirely  different  character.  The  upper  parts  of  most  ore  deposits  of 
the  metals  mentioned,  contain,  in  unglaciated  regions,  more  or  less  of  these  oxidized 
ore  minerals.  In  some  places  they  constitute  large  and  valuable  deposits,  and  a 
considerable  proportion  of  our  zinc,  lead,  copper,  and  silver  is  derived  from  them. 

Precipitation  of  Sulphides  in  Secondary  Enrichment  Zone.  —  When 
the  copper  sulphate  solution  in  its  downward  journey  reaches  the  ground- 
water  level,  no  more  oxygen  is  available;  its  metallic  content  is  then 
deposited  as  a  simple  sulphide  of  copper,  forming  what  are  called 
secondary  sulphides,  such  as  chalcocite  (CuS)  and  covellite  (CuS).  This 
deposition  takes  place  only  by  replacement  of  other  sulphide  minerals 
already  present  in  the  ore,  such  as  the  pyrite  or  chalcopyrite,  and  may 
be  expressed  by  the  following  chemical  reaction : 

14  CuSO4  +  5  FeS2  +  12  H20  =  7  Cu2S  +  5  FeSO4  +  12  H2SO4. 

The  chalcopyrite  or  other  sulphide  minerals  originally  in  the  ore 
may  also  be  replaced  by  copper  sulphides  high  in  copper  content.  By 
this  process  all  of  the  copper  originally  contained  in  the  zone  of  oxi- 
dation has  been  dissolved,  carried  down  to  the  water  level,  and  added 
to  that  already  present,  thereby  increasing  the  copper  content  and 
producing  what  is  called  the  zone  of  secondary  enrichment.  This  zone 
extends  from  the  water  level  downward  to  varying  depths,  sometimes 
to  several  hundreds  of  feet,  and  gradually  merges,  by  diminution  of 
the  secondary  sulphides,  into  the  primary  unaltered  zone.  It  is  thus 
evident  that  the  copper  content  of  the  ore  will  be  highest  in  the  zone 
of  secondary  enrichment  and  that  it  will  gradually  fall  off  with  depth 
as  the  primary  zone  is  reached. 

As  erosion  progresses  and  the  surface  and  the  water  level  are  gradu- 
ally lowered,  the  ore  previously  below  the  water  level  may  in  time  be 
exposed  to  oxidation  and  solution,  and  the  copper  may  be  carried 
again  beneath  the  new  water  level  and  redeposited.  The  process  may 
continue  until  the  copper  from  several  hundred  feet  of  the  eroded  vein 
has  been  concentrated  below.  Copper  deposits  in  which  the  primary 
ore  contains  less  than  1  per  cent  of  copper  and  is  of  too  low  a  grade  to 
be  profitably  worked  have  thus  been  enriched  to  valuable  ore  containing 


ORE    DEPOSITS  435o 

2  per  cent  or  3  per  cent  of  copper  or  even  more,  and  some  of  the  greatest 
copper  deposits  of  the  world  have  been  so  formed.  Probably  three- 
quarters  of  the  copper  produced  in  the  United  States  is  derived  from 
the  zone  of  secondary  enrichment,  and  were  it  not  for  this  process, 
such  great  mines  as  those  at  Bingham,  Utah,  at  Ely;  Nevada,  at  Miami, 
Arizona,  at  Santa  Rita,  New  Mexico,  and  others,  would  not  exist. 

Although,  for  the  purpose  of  concise  illustration,  a  copper  vein  has  been  chosen 
as  an  example,  the  same  processes  apply  to  other  metals,  and  to  other  types  of  de- 
posits than  veins.  The  changes  are  most  pronounced,  however,  with  copper  and 
silver  deposits.  The  results  of  superficial  alteration  are  to  be  found  most  commonly 
in  unglaciated  regions,  and  the  best  examples  in  this  country  occur  in  the  western 
states,  notably  in  Arizona,  New  Mexico,  Nevada,  Utah,  and  Montana. 

From  the  above  considerations  it  will  be  seen  that  deposits  affected  by  surface 
alteration,  with  the  exception  of  gold  deposits,  are  usually  barren  at  the  surface, 
grow  richer  as  the  secondary  enrichment  zone  is  reached,  and  leaner  as  the  primary 
zone  is  entered.  Also  the  character  of  ore  and  the  treatment  necessary  to  extract 
the  metal  from  the  ore  will  change  with  the  respective  zones.  The  consideratipn 
of  these  processes  of  oxidation  and  enrichment,  is,  therefore,  a  vital  necessity  for  the 
proper  understanding  and  mining  of  ore  deposits. 

Metamorphism  of  Ore  Deposits 

It  has  already  been  stated,  in  Chapter  XllI,  that  igneous  and  sedi- 
mentary rocks  are  greatly  changed  through  the  agency  of  metamorphism. 
Similarly,  ore  deposits  also  become  metamorphosed  during  the  meta- 
morphism of  the  rocks  enclosing  them.  If  the  metamorphism  is  rela- 
tively slight  it  may  be  disregarded;  but  if  it  is  extreme,  both  ore  and 
enclosing  rock  may  have  all  structural  features  destroyed  and  some  or 
all  of  the  original  minerals  recombined  and  recrystallized  into  different 
minerals.  Under  such  circumstances  virtually  all  of  the  features,  by 
which  the  different  types  may  be  recognized,  are  more  or  less  completely 
obliterated,  and  not  only  will  the  origin  of  the  deposit  be  an  extremely 
difficult  or  unsolvable  problem,  but  none  of  the  practical  deductions 
dependent  upon  a  knowledge  of  origin  may  be  utilized. 

Such  ore  deposits  occur  in  great  number  in  Sweden;  fortunately  they  are  rare 
in  the  United  States.  The  zinc  deposits  of  Franklin  Furnace,  New  Jersey,  furnish 
an  example.  Perhaps  the  most  striking  examples  are  to  be  found  in  contemporaneous 
iron  deposits  laid  down  as  beds  of  limonite  (2  Fe2O3 .  3  H2O),  or  siderite  (FeCO3), 
which  have  become  converted  into  the  hard  compact  masses  of  hematite  (Fe2O3) 
or  magnetite  (Fe3O4),  often  found  in  regions  of  metamorphic  rocks.  Here,  in  part 
at  least,  may  be  placed  the  world's  greatest  iron  deposits,  those  of  the  Lake  Superior 
region.  Originally  sediments,  they  have  later  been  changed  by  chemical  agencies 
and  by  regional  and  contact  metamorphism.  Later,  erosion  brought  them  near 
the  surface  and  rendered  them  accessible  to  exploration.  The  manifold  changes 
that  they  have  undergone  render  their  interpretation  one  of  the  most  difficult  prob- 
lems of  economic  geology. 


APPENDIX  A 

MINERALS  IMPORTANT  GEOLOGICALLY  IN  ROCKS  AND  ORES 

Introductory.  —  For  the  benefit  of  those  who  have  had  little  or 
no  previous  training  in  mineralogy,  the  following  brief  description 
of  the  chief  kinds  of  minerals,  which  have  been  mentioned  in  this 
book  and  are  important  geologically  in  rocks  and  ores,  is  appended. 
As  has  been  shown  in  the  foregoing  pages  they  are  the  ones  which 
mainly  compose  the  rocks  and  soils  and  take  part  in  important 
geological  processes.  If  time  permits,  and  the  student  is  unfamiliar 
with  them,  it  would  be  well  for  him  to  begin  his  course  with  some 
study  of  the  descriptions  here  given,  and  he  should  have  the  oppor- 
tunity of  seeing  and  comparing  representative  specimens  of  them. 

The  most  important  rock  minerals  may  be  listed  as  follows: 

Calcite,  CaCO3  Magnetite,  Fe3O4 

Chlorite,  H8(MgFe)5  Al2Si3Oi8  Micas,  H2KAl3(Si04)3,  etc. 

Clay  (Kaolin),  H4Al2Si209  Pyroxene,  CaMg(SiO3)2,  etc. 

Dolomite,  (CaMg)CO3  Quartz,  Si02 

Feldspars,  KAlSi3O8,  etc.  Rock-salt,  NaCl 

Gypsum,  CaSO4  •  2  H20  Serpentine,  H4Mg3Si209 

Hematite,  Fe2O3  Siderite,  FeC03 

Hornblende,  CaMg3(Si03)4,  etc.  Talc,  H2Mg3(SiO3)4 
Limonite,  2  Fe203  •  3  H2O 

By  observing  the  chemical  formulas  of  the  above  minerals,  some  of 
which  represent  only  one  variety  of  what  is  really  a  group,  it  will  be 
seen  that  they  comprise  silicates,  oxides,  and  carbonates  of  the  metals, 
with  one  sulphate  and  one  chloride,  which  are  of  lesser  importance. 
The  silicates,  which  are  compounds  of  metals  with  silicic  acid  (oxide 
of  silica,  Si02)  form  the  bulk  of  the  minerals  which  make  up  the 
massive  rocks  constituting  the  outer  shell  of  the  earth,  and  upon 
which  the  sedimentary  formations  rest,  and  also  of  the.  erupted 
volcanic  rocks.  Mingled  with  the  silicates  are  smaller  amounts  of 
oxides,  chiefly  those  of  iron  and  silica.  These  minerals  are  for  the 
most  part  anhydrous,  that  is,  devoid  of  combined  water.  The  car- 
bonates and  hydrated  oxides  and  the  sulphate  are  chiefly  in  the 
relatively  thin  films  which  the  sedimentary  rocks  form  on  the  sur- 
face of  the  globe.  The  salt  is  mostly  in  the  sea.  Hydrated  oxides, 

437 


438  APPENDIX 

and  silicates,  and  also  carbonates,  are  the  result  of  the  action  of 
weathering  and  the  circulation  of  water  and  other  chemical  agents 
upon  rocks  composed  of  anhydrous  minerals,  and  are  therefore  found 
in  the  outermost  zone  of  the  crust  where  these  agencies  are  at  work; 
they  occur  in  metamorphic  and  sedimentary  formations. 

Physical  Properties  of  Minerals.  —  There  are  certain  important 
physical  properties  of  minerals  which  serve  to  characterize  them  and 
to  distinguish  them  one  from  another.  These  are,  crystal-form, 
cleavage,  color,  hardness,  and  streak. 

With  regard  to  crystal-form,  the  molecules  of  minerals  in  most  cases 
have  the  property  of  so  arranging  themselves  during  growth  as  to  pro- 
duce structures  having  not  only  definite  physical  properties  but  also 
characterized  by  geometric  shapes ;  and  such  structures  are  known  as 
crystals.  Each  mineral  has  its  particular  crystal  form  which  it  en- 
deavors to  assume  if  not  interfered  with  during  its  growth.  Figures 
of  some  of  these  forms  are  shown  in  the  description  of  the  minerals 
which  follows.  They  are  not  commonly  well  developed  in  the  crystal 
grains  which  make  the  rock  particles  on  account  of  mutual  inter- 
ference during  their  growth. 

Cleavage  is  the  property  possessed  by  many  minerals  of  splitting 
or  breaking  more  or  less  perfectly  in  certain  directions  through  the 
crystal,  and  yielding  smooth  flat  surfaces.  A  familiar  example  is 
mica,  whose  usefulness  depends  on  its  perfect  cleavage.  It  varies 
greatly  in  different  minerals,  some,  like  quartz,  being  destitute  of  it. 

The  color  of  minerals  is  an  almost  obvious  property;  the  color  in 
powdered  form  may  be  quite  different  from  the  mineral  in  mass  and 
is  most  easily  seen  in  the  streak,  which  is  produced  when  a  bit  of  the 
substance  is  drawn  across  an  unglazed  porcelain  plate. 

Minerals  vary  much  in  their  hardness]  thus  gypsum  may  be 
scratched  by  the  finger  nail,  while  the  diamond  is  not  scratched  by 
any  other  substance.  Simple  means  of  testing  hardness  may  be 
found  in  the  knife-point,  a  bit  of  feldspar,  or  one  of  quartz,  each 
being  successively  harder  than  the  former. 

List  and  Description  of  Rock-minerals 
Albite,  see  under  Feldspar. 
Amphibole,  see  under  Hornblende. 
Anorthite,  see  under  Feldspar. 

Apatite.  —  Occurs  in  hexagonal  prisms,  with  ends  rounded  or  capped  by  six- 
sided  pyramids,  greenish  or  brownish  in  color;  is  easily  scratched  by  a  knife; 
has  no  good  cleavage.  It  is  a  phosphate  of  lime,  with  fluorine  (CaF)Ca4(PO4)3, 
and  although  sometimes  found  in  large  crystals  it  occurs  chiefly  in  excessively 


APPENDIX  439 

minute  microscopic  ones  distributed  through  many  kinds  of  rocks.  While  of 
no  great  geological  importance  it  fulfills  an  important  function  in  furnishing  to 
the  soil,  when  the  latter  is  made  by  rock  decay,  the  phosphorus  so  necessary  to 
plant-life  and  (through  the  plants)  to  animals  for  their  bony  structures,  etc. 

Aragonite.  —  Calcium  carbonate,  CaCO3,  like  calcite,  but  differs  from  the  latter 
in  its  crystallization,  which  is  orthorhombic.  Lacks  the  cleavage  of  calcite, 
which  readily  distinguishes  it.  Colorless,  white,  or  tinted.  While  it  sometimes 
occurs  as  a  vein  mineral  its  chief  geological  interest  is  in  the  fact  that  it  is  the 
form  of  calcium  carbonate  which  is  chiefly  deposited  by  organic  life;  thus  it  is  a 
common  component  of  many  shells,  especially  in  the  pearly  layers. 

Augite,  see  under  Pyroxene. 

Biotite,  see  under  Mica. 

Calcite,  carbonate  of  lime,  CaC03.  One  of  the  most  important 
of  geological  minerals.  Often  occurs  in  crystals,  either  pointed 
pyramidal,  or  prismatic,  or  flattened  rhombohedral  in  shape;  gen- 
erally whitish  in  color,  or  clear  transparent;  sometimes  tinted.  Is 
often  massive,  filling  fissures,  or  in  grains,  as  in  marble.  Three 
directions  of  excellent  cleavage  not  at  right  angles,  forming  rhombs. 
Easily  scratched  with  a  knife;  effervesces  readily  in  cold  acid.  Be- 
sides occurring  with  the  properties  mentioned,  as  filling  veins  and 
cavities  and  forming  marble,  calcite  in  the  form  of  minute,  not  dis- 
tinctly crystallized  granules  constitutes  the  cementing  substance  of 
the  grains  of  various  rocks,  such  as  many  sandstones;  and  in  the 
limestones,  chalks,  etc.,  makes  up  the  entire  mass  of  the  rock,  or 
nearly  so. 

Chert,  see  under  flint. 

Chlorite.  —  This  is  used  as  a  general  name  for  a  group  of  minerals 
whose  exact  chemical  nature  is  not  yet  well  known.  They  are 
hydrous  silicates  of  aluminum,  containing  ferrous  iron  and  magne- 
sium. In  outward  properties  chlorite  is  green  to  dark  green  in 
color,  and  like  mica  it  has  one  very  perfect  cleavage,  but  unlike  it 
the  cleavage  leaves  although  flexible  are  not  elastic.  Although  some- 
times occurring  in  crystals  which  are  flat  six-sided  tablets  it  is  usually 
seen  in  scaly  aggregates,  which,  although  sometimes  coarse,  are  more 
apt  to  be  fine,  producing  massive  forms.  Chlorite  is  a  secondary 
mineral  and  is  formed  by  the  alteration  and  decay  of  other  minerals 
containing  iron,  magnesia  and  alumina,  such  as  hornblende,  pyroxene, 
and  mica,  in  previously  existent  rocks.  The  dark  green  color  of 
many  igneous  rocks  is  due  to  its  formation  in  them;  thus  the  dull 
green  appearance,  and  more  or  less  soft,  earthy  character  of  many 
traps  and  basalts  is  largely  produced  by  the  change  of  some  of  the 
original  minerals  into  this  substance.  It  is  also  of  common  occur- 
rence in  the  metamorphic  rocks;  which,  as  in  green  slates,  owe  their 


440  APPENDIX 

color  to  finely  disseminated  particles  of  it;  while  in  chlorite-schist  it 
is  a  prominent  ingredient. 

Clay,  Kaolin.  —  Under  the  heading  of  clay  several  different 
substances  are  included,  compounds  of  silica,  alumina,  and  water. 
The  most  important  of  these  is  Kaolin  which  may  be  taken  as 
the  basis  of  true  clay.  Its  chemical  formula  is  H4Al2Si209  = 
A1203  •  2  Si02  •  2  H20.  It  consists  of  excessively  minute  thin  platy 
or  scaly  crystals  aggregated  in  masses.  Soft;  when  wet  coherent, 
forming  a  plastic  mass,  which  dries  coherent.  Naturally  white  in 
color,  but  often  tinted  red  or  yellow  by  iron  oxides,  or  gray  to  black 
by  organic  matter.  On  rubbing  between  the  fingers  kaolin  has  a 
smooth,  greasy  feel;  a  dry  piece  usually  adheres  to  the  tongue; 
when  dry  and  breathed  upon  it  has  a  peculiar  odor,  and  this  helps 
to  detect  its  presence.  Chiefly  formed  by  the  decay  of  feldspar. 
An  important  constituent  of  various  rocks  and  soils. 

Dolomite.  —  This  word  is  used  in  two  ways:  mineralogically,  as 
the  name  of  a  mineral,  and  geologically,  as  the  name  of  a  rock, 
largely  or  wholly  composed  of  it.  The  mineral  dolomite  is  a  com- 
pound of  one  molecule  of  carbonate  of  lime  with  one  molecule  of 
carbonate  of  magnesia  CaC03  •  MgC03.  Its  general  physical  prop- 
erties of  crystallization,  color,  hardness,  etc.,  are  so  like  those  of  cal- 
cite  (page  439)  that  it  is  not  easily  distinguished  from  it.  The  best 
test  is  by  chemical  means;  calcite  effervesces  freely  in  any  weak 
acid  when  cold;  to  produce  this  with  dolomite  the  acid  must  be  hot. 
A  further  chemical  test  for  magnesia  in  the  solution  after  eliminat- 
ing the  lime  is  confirmatory.  Many  limestones  and  marbles  are  in 
part  composed  of  dolomite,  and  may  pass  into  dolomite  in  the 
geological  sense.  The  origin  of  dolomite  has  been  discussed  on 
page  192. 

Epidote.  —  This  mineral  is  a  complex  silicate,  containing  variable  amounts  of 
alumina,  iron  and  lime  with  some  hydrogen.  It  may  be  considered  a  mixture, 
in  varying  proportions,  of  Ca2(AlOH)Al2(SiO4)3  and  Caa(FeOH)Fe2(SiO4)8.  It 
often  occurs  in  prismatic  or  bladed  crystals  with  one  perfect  cleavage,  or  in  grains, 
sometimes  aggregated  into  masses.  The  color  is  green,  from  light  to  dark,  and 
usually  of  a  yellowish-oily  tone.  Too  hard  to  be  scratched  with  a  knife.  Epi- 
dote is  a  product  of  alteration  of  other  mineral  substances  and  is  produced  in 
regional  or  contact  metamorphism,  especially  when  impure  stratified  rocks  con- 
taining calcareous  matter,  sand,  clay,  limonite,  etc.,  are  subjected  to  such  proc- 


Feldspar.  —  These  are,  perhaps,  the  most  important  of  all  minerals 
from  the  geological  standpoint  since  the  bulk  of  the  rocks  forming 
the  continental  masses  appears  to  be  most  largely  composed  of  them. 
They  are  silicates  of  alumina  with  lime,  soda,  or  potash  and  accord- 


APPENDIX 


441 


ingly  as  one  of  these  three  is  present  different  kinds  of  feldspar  are 
recognized  and  named,  as  follows: 

(a)  Orthoclase,  KAlSisOs,  silicate  of  potash  and  alumina. 
(6)  Albile,  NaAlSi3O8,  silicate  of  soda  and  alumina. 

(c)  Anorthite,  CaAl2Si2O8,  silicate  of  lime'and  alumina. 

Pure  varieties  of  feldspar,  of  the  compositions  indicated  above, 
are  mostly  confined  to  crystals  found  in  veins  and  druses  in  the 
rocks;  they  sometimes  occur  as  component  particles  of  the  rocks 
themselves,  but  are  rare;  in  most  cases  the  rock  grains  are  mixtures 
of  either  orthoclase  and  albite  on  the  one  hand,  or  of  albite  and 
anorthite  on  the  other,  and  are  then  known  as: 

(d)  Alkalic  feldspar,  (KNa) AlSi3O8,  mixtures  of  a  and  b. 

(e)  Plagioclase  feldspar  (NaAlSi3O8)a;  +  (CaAl2Si2O8)1/,  mixtures  of  b  and  c. 

In  (d)j  alkalic  feldspar,  the  potash  compound  is  usually  in  con- 
siderable excess  and  it  is  apt  to  be  referred  to  as  orthoclase,  although 
not  pure.  In  the  plagioclase  group  all  transitions  from  pure  albite 
at  one  end  to  anorthite  at  the  other  are  known;  one  mixture  in 
which  the  two  are  about  equal  is  called  labradorite. 

In  physical  properties  the  different  kinds  of  feldspar  are  much 
alike.  The  outward  crystal  form  is  not  a  matter  of  much  impor- 


A  B  CD 

Fig.  Mi.  —  Feldspar  crystals  of  several  types  of  development,  A,  B,  C,  D. 
twin  crystal  (Carlsbad  type)  is  seen. 


In  D  a 


tance,  as  it  is  rarely  seen  in  rocks,  except  in  the  phenocrysts  or  large 
embedded  crystals  of  porphyries  (see  page  327).  In  them  the  crystals 
usually  have  the  shapes  shown  -in  A,  B,  C,  and  D  of  the  adjoining 
Fig.  MI,  and  the  outlines  seen  on  a  broken  rock  surface  are  various 
sections  of  these  crystal  forms.  In  Fig.  MI,  D,  two  crystals  are  seen 
intergrown  in  what  is  known  as  a  twin  crystal;  these  may  often  also 
be  observed  in  the  rocks. 

Feldspars  possess  two  directions  of  excellent  cleavage,  one  parallel 
to  the  face  6  of  the  crystals,  the  other  parallel  to  c;  these  are  at 
right  angles  to  each  other  (orthoclase),  or  very  nearly  so  (plagio- 


442 


APPENDIX 


clase).  These  cleavages  give  the  broken  surfaces  of  the  grains  seen 
in  rocks  minutely  terraced  or  step-like  appearances,  whose  levels  in 
a  single  grain  reflect  light  simultaneously.  This  is  an  important 
means  of  distinguishing  feldspar  particles  from  quartz  grains  in  the 
rocks,  the  latter  having  no  cleavage. 

In  color,  the  feldspars  are  usually  white,  pinkish  to  deep  flesh-red, 
grayish,  or  yellowish,  very  rarely  limpid  and  colorless,  and  of  a  porce- 
lain-like appearance.  Orthoclase  is  very  often  pink  or  red,  plagio- 
clase  white,  grayish,  or  yellow;  this. rule  is  by  no  means,  however, 
an  invariable  one.  A  better  means  of  distinguishing  them,  if  it  can 
be  seen,  especially  by  aid  of  a  lens,  is  that  one  of  the  cleavage  surfaces 
of  plagioclases  is  often  ruled  by  excessively  fine  parallel  lines;  this 
does  not  occur  in  orthoclase  on  account  of  its  having  a  different 
crystallization  from  plagioclase.  The  feldspars  are  hard  and  can- 
not be  scratched  with  a  knife-point,  but  may  be  scratched  with 
quartz;  they  are  not  acted  upon  by  ordinary  acids,  properties  which 
serve  to  distinguish  them  from  some  other  cleavable  rock  minerals, 
such  as  calcite  and  dolomite. 

Under  ordinary  processes  of  weathering  the  feldspars  are  chiefly 
changed  into  clay  (kaolin);  they  may  often  be  seen  in  the  rocks 
more  or  less  altered.  They  then  become  soft  and  yield  the  clay 
odor.  In  regional  metamorphism,  especially  when  combined  with 
hydrothermal  actions,  they  are  converted  into  white  mica,  seri- 
cite,  and  take  part  in  the  formation  of  a  variety  of  other  secondary 
minerals. 

Flint.  —  This  is  not  a  rock  in  the  sense  that  it  occurs  in  extensive  independent 
formations,  like  limestone,  nor  is  it  a  definite  mineral,  like  calcite.  It  may, 
however,  be  considered  conveniently  here  among  the  minerals.  It  is  an  inti- 
mate microscopic  mixture  of  crystallized  silica,  SiO2  (quartz),  and  non-crystalline 
silica  containing  some  combined  water  (opal).  Its  color  is  dark  gray,  or  black, 
from  organic  matter;  its  hardness  is  well  known  and  like  that  of  quartz;  it  can- 
not be  scratched  by  the  knife  or  by  feldspar.  It  has  no  cleavage  but  a  con- 
choidal  fracture.  Its  use  for  striking  fire  and  in  furnishing  the  weapons  and  tools 
of  primitive  man  are  well  known.  Its  occurrence  in  concretions  and  masses  in 
chalks  and  limestones  (in  the  latter  often  called  chert)  has  been  alluded  to  in  this 
book  (page  292).  Somewhat  similar  masses  of  silica,  more  or  less  pure,  some- 
times white  or  light  gray,  and  often  differently  colored  by  iron  (yellow,  red,  or 
brown)  and  other  substances,  in  some  cases  of  similar  but  often  of  different  or 
uncertain  origin,  have  been  variously  termed  jasper,  jaspilite,  hornstone,  novaculite, 
etc.  In  places,  like  the  jaspilites  of  the  Lake  Superior  region,  or  the  novaculites 
of  Arkansas,  they  may  form  beds  of  considerable  importance. 

Garnet.  —  This  is  the  name  of  a  group  of  minerals  which  have 
the  common  chemical  formula  X3Y2(Si04)3,  and  are  thus  salts  of 
orthosilicic  acid,  H4Si04;  X  is  either  calcium,  magnesium,  or  ferrous 


APPENDIX  443 

iron,  or  mixtures  of  them  (bivalent  elements) ;  Y  is  either  aluminum, 
ferric  iron,  or  chromium,  or  mixtures  of  them  (trivalent  elements). 
The  pure  compounds  have  received 
definite  names,  thus  Fe3Al2(Si04)3  is 
known  as  almandite  and  for  the  most 
part  composes  the  common  garnet  or- 
dinarily seen  in  rocks.  Garnets  crys- 
tallize in  the  forms  shown  in  Fig.  M2, 
A  and  B.  and  crystals  of  these  forms 

f,  .      ,,  !  .    ,,  Fig.  M2.— Garnet  crystals. 

are  often  seen  in  the  rocks,  especially 

in  the  mica-schists.      The  type  A,  when  poorly  developed,  often 

appears  as  a  spherical  object  embedded  in  the  rocks. 

The  mineral  has  no  good  cleavage,  is  very  hard  so  that  it  cannot  be 
scratched  by  a  knife  or  by  feldspar,  and  varies  greatly  in  its  colors; 
common  garnet  is  deep  red  to  brownish-red  and  sometimes  black; 
yellow  and  brown  tones  are  also  common  in  garnets  containing  lime. 

Garnets  are  found  chiefly  in  metamorphic  rocks;  in  gneisses  and 
crystalline  schists.  Lime  garnets  CasA^SiOJa  (grossularite)  occur 
mostly  in  calcareous  rocks,  such  as  impure  limestones  which  have 
been  metamorphosed  by  the  action  of  intruded  igneous  masses. 

Gypsum.  —  This  mineral,  called  also  selenite,  is  the  sulphate  of 
lime  with  water,  CaS04  •  2  H2O.  It  is  sometimes  found  in  good 
crystals,  of  more  or  less  tabular  lozenge-shaped  forms,  but  as  a  rock 
constituent  it  is  massive,  foliated  with  curved  surfaces,  or  granular 
to  compact,  and  less  commonly  fibrous.  The  crystals  and  large 
crystal  grains  have  one  perfect  cleavage  by  which  it  may  be  split 
into  thin  sheets.  The  normal  color  is  white  or  colorless,  but  it  is 
often  tinted  reddish,  yellowish,  or  even  black  by  impurities.  It  is 
a  soft  mineral  and  may  be  readily  scratched  by  the  finger  nail. 

Gypsum  rock  is  widely  distributed  in  the  stratified  formations,  in 
the  form  of  extensive  beds,  often  of  great  thickness  and  is  especially 
found  with  limestones  and  shales.  It  is  indicative  of  arid  con- 
ditions prevailing  at  its  time  of  formation.  Its  use  in  making  plaster 
of  Paris  is  well  known;  this  is  done  by  heating  it  until  a  portion  olF 
the  water  is  driven  from  the  molecule;  on  mixing  with  more  water 
it  is  at  first  plastic,  when  the  water  has  again  been  taken  up  it  be- 
comes hard. 

Halite,  see  Rock-salt. 

Hematite.  —  Red  oxide  of  iron  or  ferric  oxide,  Fe2Os.  This  sub- 
stance is  found  in  several  forms,  one  of  which  is  crystalline  with  a 
steel-like  luster,  but  the  most  important  one  geologically  is  known 
as  common  red  hematite.  In  this  condition  it  is  not  crystallized, 


444 


APPENDIX 


but  is  massive,  granular  to  compact,  often  in  rounded  forms,  some- 
times earthy.  It  has  no  metallic  luster,  is  opaque  and  of  a  dark  red 
to  brown  color;  its  powder  and  streak  are  red  (distinction  from 
limonite).  It  occurs  in  sedimentary  and  metamorphic  rocks  in  beds 
and  masses,  sometimes  of  great  size,  and  furnishes  a  valuable  ore  of 
iron.  It  is  also  common  as  a  cement  of  the  grains  of  some  stratified 
rocks,  such  as  red  sandstones,  and  as  a  coloring  matter  it  is  widely 
distributed  in  all  kinds  of  rocks  and  soils,  in  some  cases  perhaps  as 
hydro-hematite,  2  Fe203  •  H20. 

In  the  crystalline  form,  as  dark  metallic  looking  specks,  it  is  widely 
distributed  in  igneous  rocks  and  certain  crystalline  schists,  but  may 
be  confused  with  magnetite,  which  see.  The  pure  mineral  contains 
70.0  per  cent  of  iron. 

Hornblende.  —  The  name  amphibole  is  used  interchangeably  with 
hornblende  and  is  given  to  an  important  group  of  rock-forming  min- 
erals, which  are  chemically  salts  of  metasilicic  acid,  H3Si03,  in  which 
the  hydrogen  has  been  replaced  by  various  metals,  such  as  calcium, 
magnesium,  or  ferrous  iron,  or  by  mixtures  of  them,  and  by  various 
radicals.  The  composition  is  too  complex  to  be  represented  very 
simply,  but  one  variety  (actinolite)  is  very  nearly  Ca(MgFe)3(Si03)4; 
common  hornblende  contains  also  alumina  and  ferric  iron. 

Hornblendes  usually  crystallize  in  prismatic  forms;    the  crystals 

are   apt   to   be    long   and   bladed, 

sometimes,  as  in  some  hornblende- 
schists,  they  may  be  very  fine  and 
'    needle-like;  in  some  cases,  as  in  cer- 
tain porphyries,  the  prisms  may  be 
short  and  stout,  Fig.  MS)  A,  or  again 
the  mineral  may  occur  in  irregular 
Fig.  MS.  — A,  Hornblende  crystal;  B,  grains  and  masses,  as  in  many  dio- 
section  of  crystal  at  right  angles  to  rites.    In  the  igneous  rocks  the  color 

vertical  axis  of  prism  showing  angles    •  11      i_i      i  •  i    i  i      i 

of  prismatic  cleavage.  |S   USUally  black  to    greenish-black ; 

in  the  metamorphic  rocks  various 
shades  of  green  to  black,  less  commonly  pale,  or  even  whitish. 

Hornblende  is  rather  hard  but  all  varieties  may  be  scratched  with 
quartz,  some  by  a  knife  point.  It  has  a  highly  perfect  cleavage 
parallel  to  the  prism  faces,  and  the  two  directions  of  cleavage  along 
the  prisms  meet  at  angles  of  125°  and  55°,  Fig.  M3,  B,  a  fact  of  im- 
portance in  helping  to  distinguish  it  from  pyroxene  (see  pyroxene). 
The  glittering  prismatic  faces  seen  on  the  blades  and  needles  of  the 
mineral  on  a  fractured  rock  surface  are  mostly  due  to  this  cleavage. 
In  small  grains  it  is  difficult  to  distinguish  from  pyroxene. 


APPENDIX  445 

Hornblende  under  proper  conditions  may  be  changed  into  serpen- 
tine, chlorite,  carbonates,  etc.,  and  by  continued  weathering  into 
limonite,  carbonates,  and  quartz. 

The  hornblendes  are  important  geological  minerals  and  occur  in 
a  great  variety  of  igneous  and  metamorphic  rocks.  They  may  be 
present  in  only  a  few  scattered  crystals,  or  to  such  an  extent  that  the 
rock,  as  in  hornblende-schist,  is  mainly  composed  of  them. 

Iron  Ores.  —  These  are  chiefly  hematite,  magnetite,  limonite,  and 
siderite  and  information  concerning  them  will  be  found  under  these 
headings. 

Kaolin.  —  This  is  the  basis  of  clay;  see  under  clay. 

Labradorite,  a  feldspar  consisting  of  about  equal  mixtures  of  albite 
(soda-feldspar)  and  anorthite  (lime-feldspar) .  Named  from  the  coast 
of  Labrador  where  it  occurs  in  large  crystals,  often  showing  a  play 
of  colors.  See  Feldspar. 

Limonite.  —  Yellow  oxide  of  iron,  2Fe203-3H20,  partly  hydrated 
ferric  oxide.  Does  not  crystallize,  but  is  found  in  earthy  formless 
masses,  which  are  sometimes  compact  and  of  rounded  shapes,  or 
stalactite-like,  and  may  exhibit  a  radiating  structure.  There  is  no 
cleavage  and  the  mineral,  while  usually  dull  or  earthy  in  appearance, 
may  in  the  compact  globular  forms  show  a  silky,  or  even  somewhat 
metallic  luster.  Color  is  usually  brown,  from  light  to  dark,  or 
brownish-yellow.  The  powder,  or  streak,  is  yellow-brown,  which 
serves  to  distinguish  it  from  hematite.  Per  cent  of  iron  59.8. 

Limonite  is  found  in  several  ways,  but  is  always  a  secondary 
mineral,  that  is,  one  formed  at  the  expense  of  previously  existent  ones 
by  weathering  and  other  agencies  which  act  chemically  upon  them. 
In  altered  igneous  and  metamorphic  rocks  it  may  be  seen  as  small 
earthy  masses  resulting  from  the  decay  of  some  previous  iron-bearing 
mineral.  It  occurs  in  the  sedimentary  strata,  see  page  422,  as  masses 
and  beds  in  compact,  globular,  or  concretionary  forms.  As  bog-iron 
ore  it  is  loose,  porous,  and  earthy.  Finally,  it  forms  the  yellow 
coloring  matter  of  many  soils,  clays,  and  sedimentary  rocks.  It  is  a 
valuable  ore  of  iron. 

Magnetite.  —  Ferrous-ferric  oxide,  FeO»Fe203,  (Fe304).  Crys- 
tallizes in  octahedrons,  sometimes  in  dodecahedrons  like  Fig.  M^B, 
page  443,  but  is  usually  seen  in  small  grains  in  the  rocks  whose  forms 
are  irregular;  sometimes  in  larger  masses.  Has  no  cleavage,  is 
brittle  and  usually  too  hard  to  be  scratched  by  a  knife.  Has  a 
metallic  luster,  sometimes  dull;  is  opaque  and  resembles  bits  of  iron 
or  steel  in  the  rocks.  Its  property  of  being  attracted  by  a  magnet 
helps  to  distinguish  it  from  other  somewhat  similar  looking  minerals. 


446  APPENDIX 

Its  powder  or  streak  is  black.  Is  widely  distributed  in  igneous  and 
some  metamorphic  rocks,  usually  in  small  grains  but  sometimes  in 
larger  masses,  especially  in  contact  metamorphic  rocks,  and  is  then 
a  valuable  ore  of  iron.  Per  cent  of  iron,  72.4. 

Micas.  —  The  micas  are  a  group  of  rock  minerals  which  are 
characterized  by  a  remarkably  perfect  cleavage  in  one  direction,  by 
means  of  which  they  may  be  split  into  almost  indefinitely  thin, 
flexible,  elastic  leaves.  They  are  silicates  of  complex  composition 
and  for  practical  purposes  may  be  divided  into  two  groups,  light 
colored  micas,  of  which  muscovite  may  be  taken  as  an  example,  and 
dark  micas,  or  biotite,  and  related  kinds.  Muscovite,  beside  the 
silica,  contains  alumina,  potash,  and  hydrogen;  biotite  contains  in 
addition  magnesia  and  iron.  Their  simpler  formulas  may  be  shown 
chemically  as  follows: 

Muscovite  =  H2KAl3(Si04)3. 
Biotite  =  (HK)2(MgFe)2(AlFe)2(Si04)3. 

They  crystallize  in  six-sided  (sometimes  four-sided)  tables,  whose 
faces  are  nearly  always  rough,  while  the  flat  bases  are  formed  by  the 
glittering  cleavage  surfaces,  Fig.  M*.     They  are 
also  often  seen  in  rocks  in   flakes,  scales,  or 
I6'      shreds,  sometimes  curled  or  bent,  with  shining 
cleavage  faces.     Muscovite  is  colorless  to  white, 

Fig.  M4.  —  Crystal  of  paje  brown,  or  greenish;  thin  leaves  are  trans- 
mica,  showing  cleavage  rru      -,  ,    1        «  . .   /.          ••  • 

parallel  to  base,  c.  parent.  The  huge  crystals  of  it  found  in  gran- 
ite-pegmatite veins  furnish  the  mica  which  is 
ordinarily  used  commercially.  In  very  minute  scales  in  the  rocks 
it  has  a  silky  appearance  and  is  known  as  sericite.  Biotite  is  black 
and  only  translucent  in  thin  scales.  All  micas  are  easily  scratched 
with  a  knife;  they  are  readily  distinguished  from  chlorite  and  talc 
by  the  elasticity  of  the  cleavage  plates,  and  this  and  the  cleavage 
distinguishes  them  from  other  rock  minerals. 

Biotite  is  found  chiefly  in  igneous  rocks,  especially  in  granites, 
syenites,  some  diorites,  in  certain  felsite  lavas  and  porphyries,  and 
in  some  trap-like  rocks  occurring  in  dikes.  Muscovite  occurs  in 
pegmatite  veins,  but  is  especially  found  in  the  metamorphic  rocks, 
as  in  gneisses,  and  is  common  in  many  crystalline  schists;  thus  in 
mica-schist  it  plays  the  chief  role.  Under  certain  conditions,  by  the 
action  of  heated  vapors  and  water,  feldspars  are  converted  into 
muscovite,  especially  the  sericite  variety. 

Muscovite,  see  micas,  above. 

Ocher.  —  This  name  is  given  to  clays  colored  deeply  red  or  yellow  by  oxides 
of  iron  (hematite  or  limonite) ;  thus  red  ocher,  yellow  ocher. 


APPENDIX 


447 


Olivine.  —  Silicate  of  magnesia,  Mg2SiO4,  the  magnesia  more  or 
less  replaced  by  ferrous  iron.  Crystals  are  rarely  well  developed  in 
rocks;  commonly  it  appears  in  grains  or  small  granular  masses. 
Color  olive  to  yellow-green;  bottle-green  very  common;  trans- 
parent to  translucent,  often  turns  reddish  and  opaque  by  oxidation 
of  the  iron.  Hard,  cannot  be  scratched  with  knife-point;  glassy 
luster. 

Olivine,  often  called  chrysolite,  occurs  almost  entirely  in  ferro- 
magnesian  igneous  rocks,  as  in  gabbros,  peridotites,  dolerites  and 
basalts.  It  is  often  seen  in  the  basalts  scattered  in  small  bottle- 
green  grains.  A  variety  of  peridotite.  composed  of  almost  pure 
olivine  occurs  in  several  regions  and  is  known  as  dunite.  The  min- 
eral alters  readily  to  serpentine  and  is  probably  the  chief  source  of 
this  substance. 

Orthoclase.  —  See  Feldspar. 

Plagioclase.  —  See  Feldspar. 

Pyroxene.  —  This  is  an  important  group  of  minerals  which,  like 
the  hornblendes,  are  salts  of  metasilicic  acid,  H3Si03,  in  which  the 
hydrogen  has  been  replaced  by  magnesium,  calcium,  and  iron,  or  by 
mixtures  of  them,  and  in  some  cases  by  sodium,  or  various  radicals. 
As  ordinarily  seen  in  the  rocks  the  light-colored  pyroxenes  are 
mostly  diopside,  CaMg(Si03)2,  with  little  or  no  iron,  while  the  dark 
or  black  kinds,  commonly  seen  in  igneous  rocks,  are  apt  to  be  the 
variety  known  as  augite.  Other 
varieties,  such  as  hypersthene 
(MgFe)SiO3,  also  occur,  but  are 
of  less  importance. 

All  pyroxenes  have  the  com- 
mon property  that   they  form 
prismatic  crystals  with  a  double 
cleavage  parallel   to   the   main 
prism  faces,  which  intersect  at 
nearly    right   angles    (93°    and 
87°).     The  crystals  are  apt  to 
be  short  and  stout,  and  well- 
formed  examples  of  augite  are  often  found  in  basaltic  porphyries 
and  lavas,  see  Fig.  M&,  A  and  B.     Pyroxenes  are  also  found  in  grains 
and  more  or  less  shapeless  masses,  as  is  common  in  gabbros  and 
dolerites. 

Pyroxenes,  while  sometimes  white,  or  even  colorless  (pure  diop- 
side), are  usually  colored  more  or  less  greenish,  light  to  dark,  while 
augite  is  black  and  opaque.  Some  can  be  just  scratched  by  a  knife- 


A  B 

Fig.  Ms.  —  A,  augite  crystal.  B,  section 
normal  to  vertical  axis,  showing  prismatic 
cleavage. 


448  APPENDIX 

point,  all  are  scratched  by  quartz.     The  luster,  which  is  often  want- 
ing, is  glassy. 

Pyroxenes  resemble  hornblendes  in  the  rocks  and  are  frequently 
difficult  or  even  impossible  to  distinguish  from  them.  The  slender- 
bladed  forms,  and  excellent  glittering  cleavage  of  hornblende  often 
aid  in  discriminating  it  from  pyroxene  whose  cleavage  is  not  so  good, 
and  whose  crystals  are  apt  to  be  shorter  and  stouter.  A  comparison 
of  the  angle  at  which  the  cleavages  meet  (pyroxene  93°,  87°  and  horn- 
blende 55°,  125°,  compare  Figs.  Af3  and  M5)  also  helps  to  distinguish 
them.  But  it  is  often  impossible,  as  in  dolerites,  to  discriminate 
between  them  by  simple  observation  and  without  other  methods  of 
testing.  Pyroxenes  are  very  important  minerals  in  the  igneous 
rocks,  especially  in  the  dark-colored  ferromagnesian  kinds,  such  as 
gabbros,  dolerites,  and  basalts,  which  are  largely  composed  of  the 
augite  variety.  In  the  feldspathic  rocks,  such  as  syenite  and  certain 
felsite  lavas,  they  sometimes  occur,  but  are  of  less  importance. 
While  they  are  found  in  some  metamorphic  rocks  they  are  of  far 
less  importance  in  this  class  than  the  hornblendes.  Indeed  by 
metamorphic  action  pyroxene  is  generally  converted  into  hornblende, 
and  much  of  the  latter  in  the  schists  owes  its  origin  to  such  conver- 
sion of  pyroxene,  when  previous  igneous  rocks  have  been  changed 
into  them. 

Quartz,  pure  silica,  Si02.  Crystallizes  in  hexagonal  prisms 
capped  by  a  six-sided  pyramid.  This  is  the  common  form  in  veins, 
druses,  geodes,  and  other  cavities  in  rocks, 
Fig.  M6, A.  In  embedded  phenocrysts  in  por- 
phyries the  prism  is  apt  to  be  small  or 
wanting,  see  Fig.  M&,B,  the  form  is  poorly 
developed  and  the  crystal  has  usually  a 
roughly  spherical  shape.  In  general  it  has 
no  outward  crystal  form  but,  as  in  igneous 

Fig.  M,  -Quartz  crystals.     r°CkS  SUCn  aS  ^anit^  &  &  HI  Small  shapeless 

masses.     In  quartz  veins  it  may  be  massive; 

in  sandstones  it  is  in  cemented  rounded  grains,  and  these  in  quartzites 
may  be  so  firmly  cemented  as  to  form  practically  massive  quartz. 

Quartz  has  no  cleavage,  but  a  conchoidal  fracture  which  helps  to 
distinguish  it  from  feldspar  in  many  rocks.  In  color  the  well-formed 
crystals  of  veins  are  usually  colorless,  or  smoky-brown,  sometimes 
purple,  but  the  rock-making  quartz  is  generally  white,  smoky,  to 
brown,  rarely  black;  massive  quartz  of  veins  is  usually  white.  The 
luster  is  glassy  to  greasy;  the  mineral  is  hard  and  cannot  be  scratched 
by  a  knife  but  scratches  glass  and  feldspar. 


APPENDIX  449 

It  is  one  of  the  commonest  of  minerals  and  occurs  in  igneous, 
sedimentary,  and  metamorphic  rocks.  With  feldspar  it  composes 
more  or  less  entirely  the  bulk  of  granites,  many  felsites,  and  is  found 
in  some  diorites.  Occurs  also  in  gneisses  and  many  schists.  In  pure 
sandstones  and  quartzites  it  may  be  almost  the  only  mineral  present. 
Excepting  limestones,  chalks,  and  marbles,  and  the  dark  heavy 
igneous  rocks,  like  dolerite  and  basalt,  its  presence  in  rocks  should, 
at  least,  be  always  suspected.  It  is  not  acted  upon  by  the  ordinary 
agents  which  decompose  other  rock  minerals  and  this  accounts  in 
part  for  its  wide  distribution,  and  also  its  being  so  commonly  one  of 
the  constituents  of  soils. 

Rock-salt,  halite,  sodium  chloride,  NaCl.  Easily  recognized  by  its  cubic 
crystals,  good  cubic  cleavage,  ready  solubility  and  saline  taste.  Colorless  and 
transparent  to  white,  translucent,  sometimes  tinted.  Common  salt  is  the  only 
chloride  in  nature  which  is  of  wide  geological  importance.  In  addition  to  its 
occurrence  in  the  sea  it  forms  beds,  sometimes  enormously  thick,  in  the  sedi- 
mentary formations,  usually  in  clays  and  shales,  and  is  generally  accompanied 
by  gypsum.  Its  presence  is  thus  indicative  of  arid  conditions  at  the  time  of  its 
deposition. 

Serpentine.  —  Hydrated  silicate  of  magnesia,  H4Mg3Si209  (2  H20  • 
3  MgO  •  2  SiO2) .  This  mineral  does  not  crystallize  but  is  generally 
found  massive,  sometimes  granular  and  not  infrequently  fibrous, 
with  fine,  silky,  flexible,  and  easily  separated  fibers.  The  color  is 
usually  green  from  light  to  dark  and  more  or  less  yellowish  or  olive 
in  tone;  sometimes  nearly  black.  Fibrous  varieties  white  to  brown, 
often  pale  brownish.  Luster  greasy,  wax-like;  has  a  greasy  feel;  soft, 
readily  scratched  or  cut  by  a  knife;  translucent  to  opaque. 

Serpentine  is  a  secondary  mineral  resulting  from  the  alteration  of 
previously  existent  magnesia-bearing  silicates,  such  as  hornblende, 
pyroxene,  and  especially  olivine.  It  appears  to  be  formed  by  the 
action  of  heated  waters  on  igneous  and  metamorphic  rocks  and  is 
found  as  masses  and  layers,  sometimes  associated  with  igneous, 
often  with  other  metamorphic  rocks.  The  fibrous  variety,  called 
also  chrysotile,  usually  occurs  in  seams,  in  massive  serpentine.  Green- 
ish and  yellowish  serpentines  are  frequently  cut  and  used  as  orna- 
mental stones  in  decoration  and  building. 

Siderite;  ferrous  carbonate,  FeCO3.  Carbonate  of  iron,  when  a  pure  mineral, 
is  extremely  like  calcite  and  dolomite,  whose  properties  should  be  consulted.  It 
crystallizes  in  a  similar  form,  has  the  same  rhombohedral  cleavage,  and  like 
them  is  soft  and  attacked  by  acids  with  effervescence;  apt  to  be  brownish  in 
color.  The  crystallized  mineral  is  not  of  great  geological  or  economic  impor- 
tance, but  massive  siderite,  either  compact  or  granular  in  character,  is  a  valuable 
iron  ore.  Beds  of  it,  more  or  less  impure  with  admixed  clay  and  limonite,  have 


450 


APPENDIX 


a  wide  distribution  and  are  known  as  clay-iron-stone.  A  variety  colored  black 
by  coaly  matter  is  known  as  black-band  ore.  In  many  places  these  deposits  are 
of  great  technical  value.  Per  cent  of  iron  in  the  pure  mineral,  48.21. 

Talc.  —  This  mineral,  like  serpentine,  is  a  secondary  silicate  of 
magnesia,  H2Mg3(Si03)4  =  H20  •  3  MgO  •  4  Si02,  produced  by  the  action 
of  circulating  heated  fluids  on  previously  existent  silicates,  such  as 
hornblendes,  pyroxenes,  and  olivine.  What  the  conditions  are  which 
determine  its  formation,  rather  than  that  of  serpentine,  are  not 
known.  The  two  are  sometimes  found  associated. 

It  is  usually  seen  in  compact  or  strongly  foliated  masses,  some- 
times in  scaly  aggregates.  Has  a  perfect  cleavage  in  one  direction 
like  mica,  but  the  cleavage  leaves,  though  flexible,  are  not  elastic. 
Has  a  mother-of-pearl  luster  and  a  soft  greasy  feel.  Softer  than 
chlorite,  marks  dark  cloth  with  a  white  streak.  Color  white  to 
greenish  or  gray;  usually  translucent. 

Talc  is  only  important  in  the  metamorphic  rocks  where  it  occurs 
in  talc  schist  and  in  the  more  massive  rock  known  as  steatite  or  soap- 
stone;  in  the  latter  it  is  usually  more  or  less  mixed  with  chlorite. 

Minerals  Important  as  Ores 

Introductory.  —  There  are  certain  minerals  which  are  not  of  importance  in  a 
broad  geological  sense,  because  they  occur  in  such  small  amounts  in  the  earth's 
crust,  but  are  yet  of  great  importance  to  man  on  account  of  the  fact  that  they 
are  the  chief  sources  of  supply  for  the  metals  used  in  commerce  and  the  arts. 
Gold  is  a  good  example  of  this,  its  value  as  a  medium  of  exchange  depending  for 
the  most  part  on  its  rarity. 

The  ores  of  iron  stand  in  an  intermediate  position,  for  they  are  not  only  im- 
portant technically,  but  also  as  rock-minerals,  as  previously  shown.  The  metals 
which  are  of  most  importance  and  whose  chief  ores  are  described  are  gold,  silver, 
lead,  copper,  and  iron. 

Gold 

This  metal  occurs  in  nature  chiefly  in  the  native  state,  as  metallic  gold.  Its 
properties  are  too  well  known  to  need  further  mention,  but  it  may  be  added 
that  it  is  easily  distinguished  from  other  substances  that  may  resemble  it  in 
color,  such  as  iron-pyrites,  by  its  softness  and  malleability,  as  it  is  rather  easily 
cut  by  the  knife  and  may  be  hammered  out  into  thin  plates. 

Silver 

This  metal  occurs  as  native  metallic  silver  and.  combined  with  sulphur  and 
also  arsenic  in  several  minerals.  An  occurrence,  imperceptible  to  the  eye,  but  of 
great  importance,  is  that  lead  and  sometimes  copper  ores  are  frequently  en- 
riched by  its  presence;  it  is  extracted  during  the  process  of  metallurgical  treat- 
ment to  which  they  are  subjected  to  obtain  the  metals. 

Native  Silver.  —  Is  distinguished  by  its  color,  softness  and  malleability,  in 
which  it  resembles  gold. 


APPENDIX  451 

Argentite,  Ag2S,  silver  sulphide,  is  perhaps  the  most  common  form  in  which 
silver  occurs  in  combination.  It  is  usually  massive,  sometimes  in  crystal  groups, 
has  a  shining  metallic  luster  on  a  fresh  surface  but  commonly  appears  black  and 
dull.  It  can  be  easily  cut,  like  lead,  with  a  knife  and  is  very  heavy.  On  fus- 
ing it  the  sulphur  burns  off  leaving  pure  silver  behind.  Percentage  of  silver  87.1. 

Lead 

Galena,  PbS,  lead  sulphide,  is  one  of  the  most  common  ores  of  lead.  In  color 
it  resembles  lead,  but  is  brittle  and  breaks  with  a  perfect  cubic  cleavage.  It  is 
usually  crystallized  in  cubic  forms  or  is  in  cleavable  masses.  It  is  very  heavy 
and  easily  fusible.  Percentage  of  lead  86.6. 

Cerussite,  PbCOs,  lead  carbonate.  Occurs  in  white  or  colorless  crystals,  or 
in  granular  whitish  crystalline  masses.  Crystals  have  a  very  high  luster.  It  is 
very  heavy  for  a  non-metallic  appearing  mineral.  It  is  easily  fusible,  yielding 
lead  and  lead-oxide;  dissolves  in  warm  dilute  nitric  acid  with  effervescence,  and 
a  little  sulphuric  acid  produces  a  precipitate  of  white  lead  sulphate  in  the  solu- 
tion. Percentage  of  lead  77.5. 

Anglesite,  PbSO4,  lead  sulphate.  Generally  in  whitish  masses,  granular  to 
compact,  but  also  occurs  in  white  to  colorless  crystals.  The  massive  varieties 
are  dull  to  earthy  in  appearance,  but  the  crystals  have  a  high  luster  and  are 
cleavable.  Like  cerussite  unusually  heavy,  but  easily  distinguished  from  it  by 
the  lack  of  effervescence  when  treated  with  nitric  acid.  Fuses  easily.  Is  often 
found  associated  with  galena,  as  an  alteration  product  of  it.  Percentage  of 
lead  68.2. 

Copper 

Native  copper,  the  metal  sometimes  occurs  as  an  ore,  especially  in  the  Lake 
Superior  region;  its  properties  need  no  further  description. 

Cuprite,  Cu2O,  copper  oxide,  or  ruby  copper  as  it  is  often  called,  usually  occurs 
in  massive  form  but  sometimes  in  crystals  showing  the  form  of  the  cube  or  octa- 
hedron. It  has  a  high  luster  in  the  crystals  to  sub-metallic  or  dull  when  massive. 
The  color  is  red  and  in  clear  crystals  ruby-like.  Easily  fusible,  tingeing  the  blow- 
pipe flame  green;  is  also  very  heavy.  Percentage  of  copper  88.8. 

Chalcopyrite,  CuFeS2,  copper  pyrites.  This  is  one  of  the  most  important  ores 
of  copper.  It  commonly  occurs  in  compact,  massive  form  and  has  a  brass  yellow 
color  and  metallic  appearance;  it  is  often  tarnished.  It  resembles  common  iron 
pyrites,  FeS2,  but  is  easily  distinguished  from  it  by  its  softness  as  it  can  be  easily 
scratched  with  a  knife.  Is  moderately  heavy.  Percentage  of  copper  34.5. 

Chalcocite,  Cu2S,  copper  glance.  Generally  found  massive,  crystals  rare. 
Has  a  conchoidal  fracture,  a  metallic  luster,  color  of  lead  and  shining  on  fresh 
surface  but  often  tarnished  and  black;  is  heavy  and  has  a  black  streak  or  powder. 
Fuses  easily;  dissolves  in  nitric  acid  and  solution  gives  the  blue  color  of  copper 
with  ammonia;  is  soft  and  easily  scratched  with  a  knife.  Common  in  the  sec- 
ondary enrichment  zones  of  veins  containing  copper.  Percentage  of  copper 
79.8. 

Malachite,  Cu(OH)2CO3,  green  carbonate  of  copper.  Occurs  in  crusts  or 
rounded  masses,  often  with  a  velvety  surface,  of  a  bright  green  color  of  varying 
shades,  and  with  a  fibrous,  radiating  structure.  Usually  dull  in  luster  and 
opaque.  Soft,  easily  scratched  with  a  knife.  Dissolves  in  acid  with  effer- 
vescence. Percentage  of  copper  57.4. 


452  APPENDIX 

Azurite,  2  CuCO3  •  Cu(OH)2,  blue  carbonate  of  copper.  Often  in  distinctly 
grouped  crystals,  also  in  rounded  radiating  masses.  Of  a  deep  azure  blue  color. 
Crystals  with  glassy  luster  and  often  transparent;  soft,  easily  scratched  with  a 
knife.  Like  malachite  dissolves  in  acid  with  effervescence.  Percentage  of 
copper  55.3. 

Iron 

The  ores  of  iron,  hematite,  limonite,  magnetite  and  siderite  have  been  already 
described  under  the  preceding  group  of  rock-minerals.  A  remaining  iron  min- 
eral of  importance  is  pyrite. 

Pyrite,  FeS2,  iron  pyrites.  Commonly  seen  in  crystals,  of  a  cubic,  or  related 
form;  crystals  often  striated  on  the  faces;  also  occurs  massive.  Of  a  brass  yellow 
color,  sometimes  tarnished;  of  a  high  metallic  luster  and  opaque.  Very  hard, 
cannot  be  scratched  with  a  knife,  which  distinguishes  it  from  chalcopyrite.  Its 
hardness  and  brittleness  distinguish  it  from  gold,  for  which  it  is  sometimes  mis- 
taken. It  is  not  used  as  an  ore  of  iron,  but  is  sometimes  mined  for  sulphur. 

Gangue  Minerals 

Common  gangue  minerals  associated  with  ores  are  quartz,  calcite,  dolomite, 
siderite  and  pyrite,  previously  described  in  this  appendix.  In  addition  two  other 
rather  common  gangue  minerals  may  be  mentioned,  barite  and  fluorite. 

Barite,  BaS04,  barium  sulphate,  commonly  called  "heavy  spar."  Generally 
in  divergent  groups  of  tabular  crystals,  also  massive,  coarsely  cleavable  or  gran- 
ular. Has  perfect  cleavages.  Generally  light  colored,  whitish,  bluish,  or  red- 
dish brown,  crystals  sometimes  transparent;  luster  glassy  or  pearly.  Heavy 
for  a  non-metallic  mineral.  Insoluble  in  acids  and  does  not  effervesce.  Char- 
acterized by  its  good  cleavage,  light  color  and  heaviness. 

Fluorite,  CaF2,  fluoride  of  calcium;  fluor-spar.  Usually  in  cubic  crystals,  but 
also  massive  and  coarse  to  fine  granular.  Color  generally  light  green  or  purple, 
rarely  white,  bluish,  etc.,  commonly  transparent  to  translucent  and  of  glassy 
luster.  Has  a  perfect  cleavage  in  four  directions  by  which  it  may  be  cleaved 
into  octahedrons.  Easily  scratched  with  a  knife;  not  a  heavy  mineral  like  barite. 
Does  not  effervesce  with  acids,  which  distinguishes  it  from  carbonates  (calcite, 
etc.). 


INDEX   TO   PART   ONE 

Asterisks  refer  to  illustrations 


Aa  lavas,  205,  206* 

Abyss,  oceanic,  112,*  114-116 

Abyssal  deposits,  113 

Accordant  streams,  54 

Acids  formed  by  plants,  171 

Aconcagua  volcano,  195 

Actinolite,  444 

Adirondack  Mountains,  267,*  268 

Agassiz,  A.,  on  coral  reefs,  188,  192 

Age,  of  earth,  estimated  by  erosion,  47 

how  estimated,  47,  56,  70,  162 
Agglomerate,  volcanic,  213,  215 
Aggradation,  48,  59 
Air  currents,  11 

deposition  by,  272 

in  ocean  water,  92 
Alaska,  earthquake  in,  248 

volcanoes  of,  199 
Albite,  325,  441 
Alga,  112,  113 

calcareous,  183,  191,  192 
in  hot-springs,  166,  234 
Alkali  deposits,  84,*  86,  168,*  169 
Alkalies,  159,  323,  328,  338,  353 
Alkaline  lakes,  86,  169 
Alluvial  cones,  64,  65* 

fans,  64,  65* 

plains,  58 

soil,  27* 
Alluvium,  58 
Almandite,  443 
Alps,  compression  of,  394 

glaciers  of,  122,*  124,  125,*  126,  129, 
137, 138* 

history  of,  382,  390-393 

orogeny  in,  382* 

thickness  of  strata  in,  295 

thrust-faulting  in,  390-393 
Alum  salts,  234 
Alumina,  201,  323,  328,  353 
Amphibole,  444 
Amphitheaters,  glacial,  139 
Anamorphism,  339 
Andalusite,  352 
Andesite,  202,  333 
Angara,  387 

Anglesite,  412,  435C,  451 
Angular  unconformities,  307,*  308,*  310 
Animals,  constructive  work  of,  182 

destructive  work  of,  174 
Anorthite,  325,  441 
Anorthosite,  331 


Antarctic  ice-cap,  128,  144,  152 
Antecedent  rivers,  76,  77,  240 
Anthracite,  284,  350 
Anticlines,  297,  298,*  301,  302,  303,  304 

and  erosion,  404-407 
Anticlinoria,  306 
Ants,  work  of,  174 
Apatite,  438 

Appal achia,  380-381,  387 
Appalachian  geosyncline,  306,  380-381 
Highlands,  267,*  268,  269 
Mountain  system,  373 
Mts.  as  example  of  geological  cycle,  403 
erosion  in,  405 

orogeny  of,  384,  385,*  386,  388 
thickness  of  strata  in,  294,  379 
Plateau,  267,*  268 
revolution,  398 
Valley  Province,  267,*  268 
Aragonite,  439 
Arctic  waters,  life  in,  113 
Argentite,  412,  451 
Argillite,  348 
Arid  regions,  11,  13,  46,  53,  82,  87,  154, 

162,  169,  172 
definition  of,  31 
erosion  in,  35 
lakes  in,  82 
Arkose,  283,  345 
Arsenic,  228 

sulphides,  234 

Artesian  wells,  157,  158-159 
Ashes,  volcanic,  114,  204,  209,  213,*  321, 

329,  343 
Asteroids,  258 
Atlantic  border,  subsidence  of,  238 

coastal  plain,  159,  267,*  268 
Atmosphere,  character  and  composition 

of,  9-12 

chemical  action  of,  on  rocks,  11 
geologic  work  of,  8-30 
constructive,  13-20 
destructive,  11-13 
lack  of,  on  moon,  10 
mechanical  action  of,  12 
movements  in,  11 
origin  of,  10 
water  vapor  in,  9 
Atmospheric  circulation,  11 

water,  solvent  action  of,  162 
Atolls,  186 

origin  of,  186-190 


453 


454 


INDEX 


Atolls,  serpuline,  191* 
Augite,  447 
Avalanches,  257 
Azurite,  412,  435B,  452 

Bacteria,  175 

antiseptic,  176 

as  lime  depositors,  113 

denitrifying,  113 

iron,  181 

Bad-lands,  35,*  36 
Bandaisan,  243 

Banding,  in  metamorphic  rocks,  341* 
Barchanes,  15,  16 
Barite,  413,  452 
Barium  sulphate,  452 
Barrell,  J.,  311,  395 
Barrier  beaches,  290 

islands,  108 

reefs,  185-190 

sand  reefs,  107* 
Barriers,  107-108 
Bars,  59,*  61,  64,  108-110,  290 
Basal  conglomerates,  307* 
Basalt,  201,  202,  205,  207    329,  333,  335, 
343,  346,  447,  448 

-porphyry,  329 
Baselevel,  69,  402 
Basement  rocks,  344 
Basin  deposits,  276 

of  Minas,  tides  in,  95 
Bathyliths,  319-320,  322,*  327,  328,  331, 

332,  351,  389,  390 
Bay  of  Fundy,  tides  in,  95 
Bays,  63 

Beach  deposits,  278 
Beaches,  106-107,  111,  309 

barrier,  290 
Bedding  planes,  271 
Bed-rock,  20,  41 

ore  deposits,  420 
Beds,  see  also  sediments  and  strata 

attitude  of,  294 

bottomset,  in  deltas,  277 

conformable,  307 

definition  of,  271 

dimensions  of,  293 

overlap  of,  294 

relative  age  of,  295 

unconformable,  307 
Benches,  101 
Bergschrund,  122,  138 
Bermuda,  a  volcanic  island,  189 
Bicarbonates,  soluble,  162 
Biotite,  325,  446 
Bituminous  coal,  284 
Black  Forest  land  mass,  387 
Block  mountains,  375,  396-398 

movements,  in  earth's  crust,  241 
Blockfaulting,  225 
Blowing  holes,  98 
Blue  muds,  of  oceans,  114 
Blue  Ridge  Province,  267,*  268 


Bog  iron  ore,  182,  445 

-moss,  177 

Bogoslov  volcanoes,  220 
Bogs,  82,  176-178,  180 
Bohemian  land  mass,  387 
Bombs,  volcanic,  204,  208,  321,  329 
Bonanzas,  434 
Bores,  tidal,  96 
Boric  acid,  203,  228 
Bornite,  412 

Bosses,  intrusive,  319,  390 
Bottomset  beds,  in  deltas,  277 
Bowlder  clay,  145,  147,  282 
Bowlders,  definition  of,  28,  273 

facetted,  144 

glacial,  132,*  145,  146* 

in  river  beds,  39 

moved  by  floods,  44,*  47,  48 

residual,  28,*  29,*  30 
Bradyseisms,  241 
Branner,  J.  C.,  174 
Breakers,  96 
Breccias,  282 

volcanic,  213,  214,  321,  329,  390 
Bridges,  natural,  163* 
British  Isles,  lava  fields  in,  221 
Brooks,  37 
Buttes,  36,  37* 
Bysmaliths,  318 

Calamine,  412,  435C 
Calcareous  algse,  183,  191,  192 
in  hot  springs,  166,  234 

sinter,  168 

tufa,  85,*  86,  186,  233 
Calcite,  25,  28,  349,  413,  437 

description  of  439 

Calcium  carbonate,  25,  46,  85,  86,  91, 
113,  159,  162,  165,  182,  188,  192, 
274,  280,  283 

fluoride,  452 

oxide,  201 

phosphate,  of  shells,  193 

sulphate,  46,  91 
Calderas,  210,  211 
Calms,  belt  of,  11 
Canyons,  6,*  51-54 
Capillary  attraction,  20,  153 
Carbon- dioxide,  159,  160,  162,  171,   172, 
175,  188,  190,  192,  201,  203 

in  atmosphere,  9,  10 

in  springs,    166 

work  of,  in  weathering,  25 
Carbon,  in  pi  ants,  175 
Carbonate  minerals,  410,  411 

rocks,  solution  of,  162 
Carbonates,  174,  338,  340 

alkaline,  161 

hydrated,  438 

of  sedimentaries,  437 
Carbonic  acid,  226,  227 

gas,  in  sea  water,  92 
solvent  action  of,  25 


INDEX 


455 


Caspian  Sea,  87 

Cataclysms,  5,  8 

Catalytic  agents  in  ore  deposition,  417 

Catskill  Mts.,  375 

Caucasus,  382,  387 

Cave  deposits,  166 

Caverns,  163-164 

Caves,  fossils  in,  167 

sea-,  98,  236* 

Cavity  fillings,  420,  424-428 
Cellulose,  175 
Cementation,  164,  172,  279 

zone  of,  165,  338,  339 
Central  Lowland,  of  United  States,  267,* 

268,  269 

Centrosphere,  396 
Cerussite,  412,  451 
Chalcocite,  412,  435C,  451 
Chalcopyrite,  412,  451 
Chalk,  191,  192,  280,  283,  349 
Chamberlin,  T.  C.,  260,  295,  387 
Charcoal,  175 

Charleston  earthquake,  248 
Chemical  action  in  ore  deposition,  417 

concentrations,  ore  deposits,  421,  435 

denudation,  161 

deposition,  164 
Chert,  292,  442 
Chief  Mt.,  Arizona,  393 
Chile,  earthquake  in,  249 
Chimborazo,  volcano,  195 
Chlorite,  340,  342,  437,  439-440 

-schist,  346,  349 
Chonoliths,  318,  390 
Christiania  Fiord,  225 
Chromite,  422,  423 
Chromium,  332 
Chrysocolla,  435B 
Chrysolite,  447 
Chrysotile,  449 
Cincinnati  axis,  381 

geanticline,  306 
Cinder  cones,  204,  209,  217 
Cinders,  volcanic,  204 
Cirques,  glacial,  139 
Citric  acid,  172 

Clay,  25,  27,  28,  29,  42,  48,  161,  172,  274, 
280,  283,  340,  342,  437,  442 

bowlder,  145,  147,  282 

description  of,  440 

effect  of  contact  metamorphism  on,  352 

metamorphosed,  345 

red,  of  ocean  bottom,  115,  116 
Clay-ironstone,  182,  350,  450 
Clarke,  F.  W.,  162 
Cleavage  cracks,  20 

in  igneous  rocks,  330 

in  metamorphic  rocks,  342-343 

in  minerals,  438 

in  mountains,  384 

plane  of,  340,  342 
Cliff  glaciers,  123 
Cliffs,  sea-,  99,  100,  101 


Climate,  arid,  11 

changes  in,  88 
causes  of,  259 

effect  of  lakes  on,  80 

effect  of  ocean  on,  92,  94 

effect  of,  on  glaciers,  136 

effect  of,  on  rock,  20-23,  204 

inferred  from  limestones,  192 

regulated  by  oceans,  92 
Coal,  284,  350 

origin  of,  180 

Coast  Range,  379,  382,  386 
Coastal  debris,  form  of,  98 

plain,  Atlantic,  159,  267,*  268 
Coast-lines,  indented,  103 

irregularities  in,  102 

submerged,  102* 
Cold,  effect  of,  on  rock,  20-23 
Colluvial  soil,  27* 
Colorado  Plateau,  265,  267,*  268,  269 

River,  47,  88 
Columbia  Plateau,  267,*  268,  269 

River  lava  fields,  221 
Columnar  structure,  in  rocks,  357 
Component  axes,  of  faulting,  363 
Compression,  and  faulting,  370-372 

in  orogeny,  382-386 
Compressive  forces,  origin  of,  393-396 
Comstock  Lode,  419 
Concentration,  chemical,  421,  435 

hydrothermal,  416 

magmatic,  415 

mechanical,  421,  433-435 

residual,  421,  435 
Concretions,  29,*  290-293 
Cone-in-crater  structure,  in  volcanoes,  212 
Cones,  alluvial,  64,  65* 

cinder,  204,  209,  217 

volcanic,  209 

Conglomerates,  280,   281-282,   289,   293, 
309,  392 

basal,  307* 

metamorphosed,  345 

of  emergence,  309 

of  submergence,  309 
Connecticut  River,  45 

Valley,  370 

Triassic  of,  377-378 
Consequent  lakes,  81 

rivers,  75,  76* 

Constructive  metamorphism,  zone  of,  339 
Contact  metamorphism,  339,  340,  350- 

353,  418,  421,  431-432 
Contemporaneous  ore  deposits,  420,  422- 

424 
Continental  deposits,  274-278 

glaciers,  124,  127,  136 

ice-caps,  189 

islands,  116 

rocks,  344 

shelves,  90,  91,  106,  107,  111,  112* 

slope,  112,*  113 
Continents,  265-266;  see  also  lands 


456 


INDEX 


Continents,  edge  of,  90 

fragmented,  116,  344 

mean  height  of,  266 
Contraction,     and    jointing,    355,    356- 

358 

Convection  currents,  in  air,  11 
"Convulsion  of  Nature,"  5 
Cooling,  and  jointing,  356-358 
Coon  Butte,  Arizona,  211 
Copper,  413,  414,  423,  435B 
Copper,  description  of,  451 

carbonates,  451,  452 

glance,  451 

minerals,  410 

native,  412,  428,  435B 

of  Bisbee,  Ariz.,  431 

of  Butte,  Mont.,  419,  427 

of  Clifton-Morenci,  Ariz.,  432 

of  Ely,  Nev.,  435D 

of  Miami,  Ariz.,  435D 

of   Santa  Rita,  N.  Mex.,  435D 

pyrites,  451 
Coquina,  192 
Coral  islands,  184-190 

limestone,  183 

reefs,  183-190 
Corals,  182,  190 

Cordillera,  North  American,  374 
Cordilleras,  363 
Corrasion,  38-41 
Corundum,  331,  332,  422 
Coseismal  lines,  248 
Cosmic  dust,  115 
Cotopaxi,  195,  202,  224 
Coulees,  53 
Country  rock,  20,  41 
Covellite,  435C 
Coves,  103 

Crater  Lake,  210,  212 
Creeks,  37 

Creep,  of  soil  and  rock,  32,  118,  170 
Crevasses,  133 
Cross,  W.,  318 
Cross-bedding,  289,  290 
Crushing,  in  contact  metamorphism,  350 
Crustification  in  veins,  426 
Crystalline  rocks,  porosity  of,  155 
Crystallization,    in    igneous    rocks,   324— 

328 

Cuestas,  407,*  408 
Cuprite,  412,  435B,  451 
Current-ripples,  289 
Currents,  air,  11 

deposition  by,  271-274 

ocean,  92-97,  114 

polar,  93,*  94 

river,  38,  40 

law  of  moving  power  of,  42 
work  of,  38,  188 

tidal,  94,  105,  110 
Cyanite,  340 
Cyclones,  11 
Cypress,  in  swamps,  178 


Dacite,  333 

Dakota  sandstone,  water  in,  159 

Daly,  R.  A.,  318 

on  coral  islands,  188 
Dams,  ice,  119 

natural,  79,  87,  146 
Dana,  J.  D.,  306 

on  coral  reefs,  187-188 
Danube  River,  45 
Darwin,  Charles,  174 

on  coral  reefs,  187-188,  189 
Datum  plane,  sea  level  as,  235 
Davis,  W.  M.,  70,  369 

on  coral  reefs,  188 
Day,  length  of,  259 

"D~eath  Gulch,"  Yellowstone  Park,  227 
Decay,  of  organisms,  25,  171-174,  175 
Deccan  lava  field,  India,  221 
Deeps,  oceanic,  91,  115,  241,  266 
Deformations,  241-242 

and  metamorphism,  336 
Degradation,  49,  71 
Deltas,  60-64 

bottomset  beds  in,  277 

deposits  in,  276-278 

in  lakes,  63 

in  seas,  63 

marine  marshes  of,  179 

peat  in,  180 

subsiding,  278 

topset  beds  in,  277,  295 
Denitrifying  bacteria,  113 
Denudation,  chemical,  161 

in  orogeny,  399 

rate  of,  47 
Deposition,  by  rivers,  58 

chemical,  164 

glacial,  138,  144-150 

in  oceans,  105-117 

of  ores,  417-419 

places  of,  274 
Deposits,  see  also  sediments 

abyssal,  113 

alkali,  84,*  86,  168,*  169 

basin,  276 

beach,  278 

cave,  166 

continental,  274-278 

current,  271-274 

deep-water,  110 

delta,  276-278 

desert,  275 

diatom,  180 

eolian,  14,  18 

fresh-water,  239 

glacial,  138,  144-150 

in  springs,  165,  167,  168,  233 

iron-ore,  181 

lime,  113,  167-168,  182,  190-193,  345 

marine,  239,  278-279 

mechanical,  272 

normal  order  of,  above  unconformity. 
309 


INDEX 


457 


Deposits,  oceanic,  105-117 

ore,  409-435 

organic,  112,  190-193,  272,  279,  439 

phosphate,  193 

river,  58,  68,  275 

sedimentary,  271-311 

siliceous,  180 

source  of,  32,  272 

subaerial,  277 

terrigenous,  112,  278 
Depression,  of  lands,  238-241 
Depth,  effect  of,  in  metamorphism,  338 
Deserts,  11,  13,  15,  17,  21,  31,  47,  162, 
169,  172 

colors  of  rocks  in,  172 

deposits  in,  275 

lakes  in,  82 

salts  in,  84 
Diabase,  332 
Diagenesis,  174 
Diamonds,  332,  422 
Diastems,  311 
Diastrophism,  235-257 

cause  of,  242-243 

sea  level  as  datum  plane  in,  235 
Diatoms,  115,  180,  234 
Dikes,  213,  314-315,  318,  322,*  327,  328, 
332,  356,  357,  397 

contact  zones  in,  351 
Diopside,  447 
Diorite,  329,  331-332,  333 

-porphyry,  329 
Dip  of  faults,  359 

of  rocks,  299-300 

of  veins,  427 
Dip-faults,  364-365 
Dip-joints,  356 
Disconformities,  311 
Disintegration    ore    deposits,    420,    421, 

432-435 

Dismal  Swamp,  178,  180,  181* 
Displacement,  of  crust,  246 

of  faults,  362,  363* 
Disseminated  ore  deposits,  428 

replacement  ore  deposits,  429 
Distributaries,  60 
Divides,  migration  of,  407 
Dolerite,  329,  331-332,  346,  447,  448 
Dolomite,  162,  192,  283,  346,  349,  413, 
437 

description  of,  440 
Domes,  lava,  206,  376 
Downfaulting,  365,*  366 
Downwarping,  240,  305 
Drainages,  interior,  82-89 
Drift,  glacial,  147,  148,  149* 
Drifts,  in  ocean  currents,  93 
Drowned  rivers,  71,*  72 

valleys,  104,  239 
Drumlins,  148,  150 
Dunes,  12,  14-17,  2o8,  290 
Dunite,  332,  447 
Dust,  28 


Dust,  cosmic,  115 

volcanic,  19,  203,  204,  321 
Dutton,  C.  E.,  394 
Dynamical  Geology,  4,  8 
Dynamo-metamorphism,  337 

Earth,  age  of,  how  estimated,  47,  56,  70, 
162 

axis  of,  259 

crust  of,  see  lithosphere 

density  of,  260 

distance  of,  from  sun,  258 

elasticity  of,  260 

energy  of,  from  sun,  194 

interior  heat  of,  194,  222,  261-264,  280 

neighbors  of,  258 

origin  of,  10,  263-264 

pressure  of,  261 

rigidity  of,  260 

shape  of,  90,  259,  266-268 

shrinkage,  222,  223,  260,  394,  395 

structure  of,  258-270,  296-311 
Earthquakes,  8,  243-257 

cause  of,  243 

centrum  of,  248 

epicenter  of,  248 

examples  of,  248 

focal  point  of,  248 

geologic  effects  of,  257 

methods  of  recording,  253 

seat  of  shock  in,  254 

submarine,  252 

tectonic,  246 

tremors  of,  254 

waves  of,  247,  255 
Earthworms,  work  of,  174 
Economic  Geology,  definition  of,  4 
Eifel,  volcanic,  211 
Electrolytic    action    in    ore    deposition, 

417 

Elements,  disintegration  of,  395 
Elevation  of  lands,  236-238,  240-241 
Emergence,  conglomerates  of,  309 

of  lands,  236-241 

Endomorphic    effect,    of    contact    meta- 
morphism, 350 
Englacial  material,  134 
Enrichment  of  ore  deposits,  435-435  D 
Eolian  deposits,  14,  18 
Epeiric  seas,  111-112 
Epeirogenic  movements,  241 
Epicenter,  of  earthquakes,  248 
Epicontinental  seas,  90 
Epidote,  440 
Equatorial  current,  93 
Erosion,  32-57 

and  anticlines,  404-407 

and  synclines,  404-407 

by  waves,  98-102 

effect  of  vegetation  on,  33,  34* 

glacial,  138-140 

in  arid  regions,  35 

in  mountains,  374 


458 


INDEX 


Erosion,  in  orogeny,  399-408 

in  volcanoes,  214 

power,  law  of,  in  rivers,  40 

rate  of,  46,  47 

remnants  of,  36 
Erratics,  132,*  145,  146* 
Eruptions,  volcanic,  194,  197-200 
explosive,  197,  321 
quiet,  199,  202,  209,  320 
Escarpments,  407,*  408 
Eskers,  149,  150* 
Estuaries,  63,  72,  103-105 
Ethmoliths,  318 
"Everlasting  hills,"  373 
Exomorphic  effect,  of  contact  metamor- 

phism,  350,  351 

Explosion  pits,  in  volcanoes,  211 
Extrusions  of  magma,  389 
Extrusive  sheets,  320,  322,*  332 

Facetted  bowlders,  144 

spurs,  142,  143* 
Fans,  alluvial,  64,  65* 
Fault-breccias,  360 
Faulting,  8,  358-372;   see  also  faults 

as  cause  of  lake  origin,  79 

block-,  377 

components  of,  362 

gravitative,  371 

in  Appalachians,  365 

in  Great  Basin,  366 

in  mountains,  375-378,  384 

in  Plateau  region,  365 

magnitude  of,  365 

topographic  results  of,  367 
Fault-line  scarps,  369,  370 
obsequent,  369 
resequent,  369 
Fault-lines,  248 

Faults,  244,  272,*  303,  358-372;   see  also 
faulting 

compression,  361 

dip-,  364-365 

dip  of,  359,  364-365 

displacement  of,  362,  363* 

erosion  of,  370 

hanging  wall  of,  360 

heave  of,  363 

normal,  361,  362,  365,*  371 

oblique,  364 

origin  of,  370 

reverse,  361,  362,  366,  367,  371 

rotary,  365 

shove  of,  247,*  363,  364 

slip  of,  362,  363* 

step-,  359 

strike-,  364-365 

strike  of,  359,  364-365 

tension,  361 

throw  of,  363 

thrust-,  366-367,  388,  389,  390-393 
Fault-scarps,  365,  367-370,  376 

-surface,  359-362 


Feldspar,  25,  28,  161,  274,  325,  328,  329, 
330,  331,  332,  333,  340,  345,  346, 
437,  440-442 

Felsite,  205,  207,  329,  333,  343,  346,  347, 
350,  356,  449 

-magmas,  333 

-porphyry,  329,  333 
Ferric  oxide,  172 

sulphate,  435A 
Ferrous  carbonate,  162,  172 

oxide,  172 
Fiji,  184,  344 

Fiords,  102,*  103-105,  144 
Firn,  see  neve 
Fissility,  283 
Fissure  springs,  157,  228 

veins,  425-428 
Fissures,  163,  354,  355 

lava  flows  from,  320 

water  in,  153 
Flats,  tidal,  108 
Flint,  192,  292,  442 
Flood-plain  scrolls,  75 
Flood-plains,  58,  148,  154* 
Floods,  43,  64,  80 
Flo  wage,  zone  of,  415 
Fluorine,  in  metamorphism,  338 
Fluorite,  452 
Fluorspar,  452 

Folded  strata,  erosion  in,  404-407 
Folding,  see  also  folds 

as  cause  of  mountains,  375 

zone  of,  385* 
Folds,  296-306 

broken,  303 

closed,  303,*  304 

inclined,  302 

isoclinal,  304 

open,  303,*  304 

overturned,  303 

recumbent,  303 

unsymmetric,  303 
Foodstuffs,  in  seas  and  oceans,  113 
Foot-prints,  in  stratified  rocks,  285 

-wall,  of  faults,  360 

of  veins,  427 

Foraminifera,  115,*  190,  191,  192 
Foreset  beds,  in  deltas,  277,  290,  295 
Forests,  effect  of  removal  of,  34 

submerged,  238 
Formations,  definition  of,  272 

lenticular,  293 

sedimentary,  271-311 

universal,  293 

water-bearing,  154 
Fosse,  366 
Fossils,  285 

as  evidence  of  earth  upheaval,  236 

imprints  of,  in  concretions,  291 

in  caves,  167 

in  chalk,  192 

in  metamorphic  rocks,  335,  343 

in  tuffs,  321 


INDEX 


459 


Foyaite,  331 

Fracture  lines,  volcanoes  on,  219 

zone  of,  338 
Fractures,  354-358,  389 

depth  of,  244 

Frontal  aprons,  glacial,  149,  150 
Frost,  effect  of,  on  rock,  20-23,  118 
Fumaroles,  226-228 
Funafuti,  187 

Gabbro,  329,  331-332,  447,  448 
Galena,  412,  451 

argentiferous,  411,  430 
Ganges  River,  47,  63 
Gangue  minerals,  409,  412,  413,  452 
Garnet,  340,  346,  353 

description  of,  442 
Gases,  in  earth's  interior,  223-225 

in  lavas,  207 

in  magmas,  201,  202 

in  metamorphism,  337 

in  volcanoes,  197,  202-203 
Geanticlines,  304-306,  381,  391,  396,  398 
Geikie,  J.,  387 
Gem-stones,  434 

Geography,  ancient,  see  Paleogeography 
Geologic  agencies,  8 

cycles,  399,  403-404 

history,  revealed,  308 

processes,  9 

records,  3 

sciences,  3 

time,  how  estimated,  47,  56,  70,  162 

work,  rate  of,  8 
Geology,  definition  of,  3—5 

methods  of  study  of,  5-7 

processes  of,  8-9 

subdivisions  of,  3—5 
Geosynclines,    304-306,    380-381,    385,* 

386,  396 

Geyserite,  227,*  229,*  233 
Geysers,  230-233 
Giant's  Causeway,  357 
Gilbert,  G.  K.,  318 
Glacial  amphitheaters,  139 

bowlders,  132,*  145,  146* 

cirques,  139 

deposits,  138,  144-150 

drift,  147,  148,  149* 

frontal  aprons,  149,  150 

ice,  135,  136* 

lakes,  79,  81,  146,  147* 

outwash  plains,  149 

rivers,  135 

stria?,  140,  141,*  144 

till,  145,  147,  282 

troughs,  submarine,  144 

valleys,  141-142 
Glaciation,  140;    see  also  glaciers 

by  ice-caps,  142 
Glacierets,  123 
Glaciers,  104,  120-152 

advance  of,  136 


Glaciers,  Alpine,  124 

as  source  of  water  for  oceans,  189,  190 

characters  of,  120-144 

cliff,  123 

continental,  124,  127,  136 

drainage  by,  135 

effect  of  climate  on,  136 

geographic  distribution  of,  126 

geologic  work  of,  138-144 

hanging,  123 

lower  limit  of,  124 

moraines,    125,*    133,    134-135,    137,* 
144-145,  147,  148,  150 

movement  of,  122,  129-133,  136-138 

of  Alps,  122,*  124,  125,*  126,  129,  137, 
138* 

of  New  Zealand,  124 

piedmont,  124,  126,  127* 

plucking  by,  134,  138,  139 

recession  of,  136 

reconstructed,  123 

rock-,  118,  119,*  170 

surface  of,  133 

tide-water,  127 

transportation  by,  138,  144 

types  of,  124 

valley,  124,  125,*  135* 
Glass,  326 
Glass-porphyry,  329 
Glauconite,  114 

Gneiss,  331,  341,*  345,  346-347,  358,  389 
Gold,  44,  333,  411,  413,  414,  450 

minerals,  410 

mines,  413 

native,  412 

of  Alaska,  434 

of  Australia,  428,  434 

of  California,  427,  434 

of  Cripple  Creek,  Colo.,  410 

of  Hedley,  B.  C.,  432 

of  Klondyke,  434 

of  Nova  Scotia,  428 

placer  mining  of,  44,  434,  435 

-quartz  veins,  425 

veins  of  California,  427 
Gorges,  6,  51-54,  55 
Gossan,  435B 
Gouge,  in  ores,  426* 
Graben,  242,  366 
Graham's  Island,  220 
Grand  Canyon,  52,*  53,  220,  375 
Granite,  28,  155,  329,  331,  333,  346,  350, 
351,  449 

core,  in  mountains,  389 

-porphyry,  329 
Graphite,  350 
Gravel,  28,  47,  149,  273,  280 

metamorphosed,  345 

water  in,  155 

Gravity,  as  cause  of  earth's  heat,  222 
Graywacke,  283 
Great  Barrier  Reef,  183,*  186 
Great  Basin,  47,  82,  267,  268,  269 


460 


INDEX 


Great  Basin,  block  mountains  in,  397 

fault-line  scarps  in,  369 

ranges,  377 

Great  Ice  Barrier,  128,  144,  152 
Great  Lakes,  tilting  of,  240 
Great  Plains,  267,*  268,  269 
Great  Salt  Lake,  85,  87,  101,  293 
Green  muds  of  oceans,  114 
Greenland  ice-cap,  128,  129,  152 
Grossularite,  443 
Ground-swells,  in  seas,  97 
Ground-water,    153,    155,    159-169,  233, 

338,  339 
Guano,  193 
Gulches,  32,  37 
Gulf  of  California,  88 
Gulf  Stream,  93 
Gullies,  32,  35,  36,  37 
Gypsum,  161,  169,  173,  174,  284,  437 
description  of,  443 

Hade,  of  dikes,  314 

of  faults,  359 
Halite,  449 
Hanging  valleys,  141,  142,*  143* 

wall,  of  faults,  360 

of  veins,  427 

Hawaii,  116,  184,  195,  200,  220 
Headlands,  94,  103 
Heat,  effect  of,  on  rock,  20-23 

in  metamorphism,  337,  350 

in  mines,  261 

in  orogeny,  393-396 

in  tunnels,  262 

of  earth's  interior,  194,  222,  261-264, 

280 

Heave,  of  faults,  363 
Hedin,  Sven,  17 
Heim,  A.,  394 
Helium,  395 
Hematite,  350,  412,  413,  437 

Clinton,  424 

description  of,  443 
High  wood  Mts.,  Montana,  220 
Historical  Geology,  3;  5 
Hogbacks,  406,*  407 
Homoclines,  304 

Hornblende,  325,  329,  331,  332,  340,  342, 
346,  437 

description  of,  444 

-schist,  346,  349 
Hornfels,  352 
Hornstone,  442 
Horses,  in  veins,  427 
Horsts,  242,  371,  386,  395,  396 
Hot  Springs,  Va.,  157 
Hot-springs,  166,  226,  228-234,  416 
Hudson  River,  239 
Humboldt  glacier,  152 
Humic  acid,  171 
Humid  regions,  13,  31,  46,  53,  82,  87,  154, 

162,  169,  176,  276 
Humus,   29,  171,  172 


Hydrochloric  acid,  226 

in  volcanoes,  201,  203 
Hydrofluoric  acid,  in  volcanoes,  203 
Hydrogen,  in  volcanoes,  203 

sulphide,  226 
Hydro-mica-schist,  348 
Hydrosphere,  155 
Hydrostatic  pressure,  157 
Hydro  thermal  concentration,  416 
Hypersthene,  447 

Ice,  as  a  geologic  agent,  18,  118-152 
behavior  of,  under  pressure,  132 
dams,  119 

floating,  geological  work  of,  152 
glacial,  veins  and  layers  in,  135,  136* 
gliding  planes  in,  132 
in  lakes,  119 
in  soil,  118 
plateaus  of,  126 
ramparts,  120 
river,  119 
seas  of,  128 
specific  gravity  of,  151 
structure  of,  130,  131 
tongues,  123 
viscosity  of,  131 
work  of,  18,  118-152 
Icebergs,  124,  127,  128,  151-152 
Ice-caps,   124,    127,    128,    129,    136,    144 

152 

Antarctic,  128,  144,  152 
continental,  189 
deposits  of,  147-150 
Greenland,  128,  129,  152 
Ice-cliffs,  127,  128* 
Ice-fall,  133 
Iceland,  221 
Iddings,  J.  P.,  318 
Igneous  agencies,  194-234,  378 

work  of,  in  mountains,  389-390 
ore  deposits,  420,  422-423 
rocks,  312-334,  378,  389 
age  of,  322 

as  source  of  sedimentaries,  345 
characters  of,  312 
classification  of,  322-334 
cleavage  in,  330 
composition  of,  323 
crystallization  in,  324-328 
extrusive,  320-322 
feldspathic,  329 
ferromagnesian,  329 
how  studied,  330 
joints  in,  356 
lime  in,  323 

occurrences  of,  313-320 
salts  in,  323 

texture  of,  312,  325-328 
Illecillewat  glacier,  137 
Imperial  Valley,  88 
Inland  drainage,  82-89 
Inlets,  108,  109 


INDEX 


461 


Interior  Basin,  268 

Highlands,  267,*  269 

Low  Plateaus,  267,*  269 

Plains,  267,*  269 

Intermediate  continental  slope,  112,*  113 
Intermontane  Plateaus,  267,*  269 
Intrusions,  contact  zones  in,  351 

of  magma,  389 
Intrusive  bosses,  319,  390 

sheets,  315,  318,  322,*  327,  328,  332, 

357,  390,  397 
Iron,  328,  353,  412,  413,  414,  452 

bacteria,  181 

carbonate,  168 

deposits,  sedimentary,  416 

minerals,  410 

mines,  413 

of  Lake  Superior,  435D 

of  Lorraine,  France,  424 

ore,  284,  325,  332,  346,  350 
and  life,  182 
bog,  182,  445 

oxides,  168,  172,  201,  280 
in  igneous  rocks,  323 

pyrites,  452 
"Iron  hat,"  435B 
Islands,  105,  116-117 

barrier,  108 

continental,  116 

coral,  184-190 

oceanic,  116,  374 

volcanic,  116,  185,  187,  188,  189,  219, 

220,  265 

Isogeotherms,  262 
Isostasy,  239,  264-265,  380,  395 
Izalco,  volcano,  217 

Japanese  current,  93,*  94 
Jaspilite,  292,  442 
Jetties,  64 
Joints,  163,  354-358 

columnar,  357 

compressional,  356 

in  mines,  354,  356 

in  tunnels,  356 

master-,  356 

strike-,  356 

tensional,  356 
Jorullo,  volcano,  216 
Jupiter,  258 

Serapis  temple,  237 
Jura  Mts.,  388,  392,  401,  405 
Juvenile  water,  166,  224-225,  416 

Kames,  149,  150 

Kaolin,  25,  161,  274,  342,  437,  442;      see 

also  clay 

description  of,  440 
Katamorphism,  339 
"Kettles,"  147,  149 
Kilauea,  200,  202,  203,  205,  225 
Kingston  earthquake,  249 
Krakatoa,  197.  202,  210,  243 


La  Caldera,  Canary  Islands,  210 

Labradorite,  331,  441,  445 

Laccoliths,  315,  316-318,  322,*  328,  332, 

356,  374,  376,  390,  397 
Lagoons,  107,*  108,  185,  186,  188,  189, 

190 

Lake  Bonneville,  87,  88* 
Drummond,  178 
Lahontan,  88 
Superior  geosyncline,  305 

iron  ores,  435D 
Temiskaming,  240 
terraces,  87,*  88 
Lakes,  60,  79-89,  108,  109 
alkaline,  86,  169 
consequent,  81 
deltas  in,  63 
duration  of,  80 
effect  of,  on  climate,  80 
filling  of,  by  peat,  176,  177* 
glacial,  79,  81,  146,  147* 
ice  in,  119 
in  arid  regions,  82 
in  calderas,  211 
in  humid  regions,  82 
lava,  200,  202 
origin  of,  79 
relic,  80,  87 
rock-basin,  146 
salt,  83,*  84-89,  169 
sedimentation  in,  80 
temporary,  82* 
tides  in,  96 
tilting  of,  240 
Laminae,  272 

Lamination,  oblique,  see  cross-bedding 
Land  masses,  ancient,  387 
plants,  in  sea,  113 
waste,  32,  272 

annual  removal  of,  by  rivers,  162 
Lands;   see  also  continents 
as  positive  elements,  243 
mean  height  of,  266 
upheaval  of,  236-241 
warping  of,  240 
Landslides,  118,  169,  257 
Lapilli,  volcanic,  204,  208,  209,  321 
Laramide  Mountain  system,  373 

revolution,  398 
Lassen's  Peak,  volcano,  217 
Lateral  pressure,  in  orogeny,  382 
Laurentian  Plateau,  267*,  268 
Lava,  194,  201,  204-209,  213-214,  314, 

347 

ao,  205,  206* 
andesite,  202 
basalt,  see  basalt 
cones,  209 

crystallization  of,  208 
domes,  206,  376 
felsite,  see  felsite 
fields,  of  British  Isles,  221 
of  northwestern  U.  S..  221 


462 


INDEX 


Lava  flows,  320,  328,  332,  351,  357,  390 

submarine,  320 
gases  in,  207 
glass-like,  208 
lakes,  200,  202 
pahoehoe,  205,  206* 
plateaus,  221 
rate  of  flow  of,  205 
Lead,  333,  412,  413,  414.  450 
carbonate,  451 
description  of,  451 
minerals,  410 

of  Coeur  d'Alene  district,  Idaho,  430 
of  Leadville,  Colo.,  430 
of  Mississippi  Valley,  416,  428 
of  Wisconsin,  428 
sulphate,  451 
sulphide,  451 
LeConte,  J.,  386,  403 
Levees,  artificial,  64 

natural,  59 

Levelling  agencies,  153 
Life,  destruction  of,  by  tsunamis,  252 
elements  in,  113 
geologic  work  of,  171-193 
in  waters,  113 
Lignite,  284 
Lime,  201 

deposits,  113,  190-193 
metamorphosed,  345 
in  igneous  rocks,  323 
Limestone,  25,  162,  191,  280,  281,  283- 

284,  293,  349,  355 
as  climatic  indicator,  192 
caverns  in,  164 
coral,  183 

effect  of  contact  metamorphism  on,  352 
metamorphosed,  345 
shell,  190,  192 
Limonite,  172,  181,  182,  284,  350,  412, 

413,  437 

description  of,  445 
Liquids,  in  metamorphism,  337,  350 
Lithodomus,  236 
Lithophysse,  334 
Lithosphere,  222-226,  269 
block  movements  in,  241 
chief  minerals  of,  437 
density  of,  260 
displacements  in,  246 
fracturing  of,  354-372 
movements  in,  235-257,  336,  373-408 
negative,  187,  188,  238-243,  244,  380 
positive,  236-238,  240-241,  242,  243, 

386,  395,  396 

negative  elements  in,  187,  242,  388 
rigidity  of,  260 
thickness  of,  256 
zones  in,  338 
zones  of  weakness  in,  251 
Little  Rocky  Mts.,  Montana,  397 
Littoral  region,  111 
Loam,  29 


Loam,  porosity  of,  155 
Lode  ore  deposits,  420 
Loess,  12,  17-19,  272 

fossils  in,  18 

in  China,  18,  19 
Lopoliths,  318 
"Lost  interval,"  308 
Lower  California  Province,  267,*  269 
Luray  Cavern,  Va.,  164 

Maars,  211 

McConnell,  R.  G.,  386 
Magmas,  200-202,  212,  214,  312-334,  350, 
351,  389,  390,  397 

cause  of  ascension  of,  225-226 

composition  of,  201 

cooling  of,  324 

gases  in,  201,  202 

in  metamorphism,  337 

intrusive,  215 

origin  of,  223-224 

relation  of,  to  volcanic  eruptions,  202 

source  of  heat  of,  222 
Magmatic  concentration,  415 

differentiation,  422 

water,  224-225,  228,  416 
Magnesia,  201,  283,  323,  328,  349 
Magnesium  carbonate,  46,  162 

chloride,  91 

silicates,  349 

sulphate,  91,  169 
Magnetite,  325,  350,  412,  423,  437 

description  of,  445 

of  Sweden,  423 
Malachite,  412,  435B,  451 
Malaspina  glacier,  126,  127* 
Mammoth  Cave,  164 
Mammoth  Hotsprings,  165,*  166 
Man,  as  a  geologic  agent,  174 
Mangroves,  in  marshes,  179 
Marble,  280,  335,  340,  345,  346,  349-350, 

352 

Marl,  29 
Marsh  gas,  175 
Marshes,  82 

salt,  108,  178-179 

tidal,  104,*  105 

Mashing,  in  metamorphism,  337,  350 
Master  joints,  356 

streams,  76,  77,  404 
Matterhorn,  393 
Mauna  Loa,  200,  205,  209,  225 
Meanders,  64,  65,*  66,*  74,  75 
Mechanical    concentrations    in    ore    de- 
posits, 421,  433-435 
Mediterraneans,  112 
Mercury,  419 
Mesas,  36 

Messina  earthquake,  249 
Metallic  carbonates,  437 
chlorides,  437 
content  of  ore,  414,  415 
oxides,  437 


INDEX 


463 


Metallic  silicates,  437 

sulphates,  437 

Metallurgy,  relation  of,  to  ore,  410 
Metals,  409-435 

combinations  of,  411 
table  of,  410 

concentration  of,  415 

native,  411 

solution  of,  in  oxidized  zone,  435A 

source  of,  415 

transportation  of,  415 
Metamorphic  rocks,  335-353 

age  of,  344 

banding  in,  341* 

classification  of,  345-350 

cleavage  in,  342-343 

igneous,  345,  346 

in  mountains,  344 

joints  in,  358 

minerals  of,  340 

places  of  occurrence  of,  344 

texture  of,  340-342 
Metamorphism,  335-353,  389 

agencies  in,  336 

constructive,  zone  of,  339 

contact,  339,  340,  350-353 
of  ores,  421,  431-432 

definition  of,  335 

dynamic,  337 

gases  in,  337 

heat  in,  337,  350 

hydrothermal,  349 

regional,  339 

shearing  in,  337 

solutions  in,  337 

water  in,  337 

Metamorphosed  ore  deposits,  422,  435D 
Meteoric  waters,  416 
Meteorites,  9,  11,  211,  259,  261 
Meteorology,  definition  of,  4 
Mica,  325,  328,  331,  332,  333,  340,  342, 
346,  347,  348,  437,  446 

-schist,  346,  347 
Mineral  springs,  157 
Mineralization,  153 
Mineralogy,  definition  of,  4 
Minerals,  324-325,  350,  351,  437-452 

carbonate,  410 

cleavage  of,  438 

copper,  410 

crystal-form  of,  438 

ferromagnesian,  325,  329 

gangue,  409,  412,  413,  452 

gold,  410 

hardness  of,  438 

"high- temperature,"  419 

iron,  410 

lead,  410 

native,  410 

of  metamorphic  rocks,  340 

ore,  409-435,  450-451 
definition  of,  409,  410 

oxide,  410 


Minerals,  physical  properties  of,  438 
rock-,  description  of,  438-452 
silicate,  410 
streak  of,  438 
sulphate,  410 
sulphide,  410 
Mines,  gold,  413 
heat  in,  261 
iron,  413 

joints  in,  354,  356 
water  in,  154,  155 
Mining  ore  deposits,  409-435 

placer,  44,  433-435 

Mississippi  Basin,  rate  of  rock  removal 
in,  47 

size  of  plain  of,  5 
River,  annual  discharge  of,  46 
delta  of,  62,*  63 
sediment  carried  by,  45 
Mofets,  227 
Moles,  work  of,  174 
Molluscs,  nature  of  shell  of,  190 
Monadnocks,  71,  402 

Monoclines,  304,  306* " 

Mont  Pelee,  198,  202,  207,  243 

Monte  Somma,  199,  212 

Moon,  10,  94,  212 

Moraines,  125,*  133,  134-135,  137,*  144- 

145,  147,  150 
recessional,  148 
Moss,  bog-,  177 

sphagnum,  177 
Mt.  Adams,  195 
Baker,  195 

Etna,  197,  203,  210,  216 
Hood,  195,  210 
Mazama,  211 
Rainier,  195,  210 
Shasta,  195,  210,  227 
Mountain  ranges,   336,   373,   378-382 

folded,  378-382 
systems,  373 

Mountain-making,  see  orogeny 
Mountains,  266;   see  also  orogeny 
and  isostasy,  265 
block,  375,  396-398 
complex,  378,  389-393 
definition  of,  373 
domed,  374 
due  to  extrusions,  374 
due  to  intrusions,  374 
erosion  of,  54,  399-408 
folded,  375,  376,  378-382 
granite  core  in,  389 
grouping  of,  373 
history  of,  379-404 

erogenic  period,  379,  382-399 
post-orogenic  period,  399-408 
pre-orogenic  period,  379-382,  385* 
mature,  401,  403 
metamorphic  rocks  in,  344 
of  erosion,  54,  374 
old,  401,  403 


464 


INDEX 


Mountains,  origin  of,  374-378,  398 

rejuvenated,  403 

roots  of,  403 

structure  of,  373-408 

youthful,  401,  403 
Muck,  29 
Mud  geysers,  230 

volcanoes,  229,*  230 
Mud-cracks,  286,  287,  358 
"Mud-pots,"  in  springs,  230 
Muds,  28,  112,  274,  280,  283,  342 

blue,  114 

liquid,  on  volcanoes,  214 

oceanic,  114 

Muir  Glacier,  127,  128,*  138 
Murray,  John,  on  coral  islands,  188,  189 
Muscovite,  446 

Native  copper,  412,  428,  435B 

gold,  412 

minerals,  410,  411 

silver,  412,  435C 
Natural  bridges,  163* 

dams,  79,  87,  146 
Nebular  hypothesis,  263 
Necks,  volcanic,  215,  318,  319,  322,*  328, 

332,  358,  374 

Negative  elements,   in  lithosphere,  187, 
242,  388 

movements,  of  lithosphere,   187,   188, 

238-243,  244,  380 
Nephelite,  325,  331 

-syenite,  331,  333 
Neve,  121 
New  Caledonia,  184,  185,  344 

England  Province,  267,*  268 

Madrid,  Mo.,  subsidence,  240,  244 

Zealand,  117,  124 
Niagara  Falls,  55-56 
Nickel,  332,  422,  423 

of  Norway,  423 

of  Sudbury,  Ont.,  423 
Nile  delta,  60,*  63 

River,  sediment  carried  by,  45 
Nitrogen,  in  atmosphere,  9 

in  life,  113 

in  plants,  175 

in  volcanoes,  203 
Nonconformities,  311 
North  America,  character  of,  268 

height  of,  above  sea,  47 
Novaculite,  292,  442 
Nuggets,  434 
Nullipores,  184,  191 
Nunataks,  128 

Obsidian,  208,  329,  333-334 

Cliff,  209 
Oceanic  islands,  116,  374 

level,  rise  of,  188 
Oceans,  abysses  in,  112,*  114-116 

basins,  113 

bottom  of,  91,  115 


Oceans,  constructive  work  of,  105-117 

currents,  92-97,  114 

deposits  in,  105-117 

depth  of,  90,  91,  115,  241,  266 

destructive  work  of,  97-105 

erosion  by,  97 

extraction  of  water  for  glaciers,  189 

functions  of,  92 

general  characters  of,  90 

increase  of,  236 

muds,  114 

negative  elements,  243 

oozes,  115 

permanence  of,  116 

plant  life  in,  113,  114 

red  clays,  115,  116 

regulator  of  climate,  92 

salts  in,  91 

submergence  by,  106,  308-309 

subsiding  areas,  187,  188,  243 

temperature  of,  in  abyss,  115 

water,  chemical  composition  of,  91 

work  of,  90-117 

zones,  diagram  of,  112 
Ocher,  172,  446 

"Old  Faithful"  geyser,  231,*  232 
Olivine,  325,  332,  349,  447 
Onyx,  167 
Oolites,  293 
Oozes,  115 
Ore,  black-band,  450 

chambers  of,  427 

constitution  of,  413 

definition  of,  409,  410 

deposition,  417-419 

magmatic,  of  Adirondacks,  423 

metallic  content  of,  414,  415 

minerals,  409-435 

shoots,  426 

tenor  of,  414 
Ore  deposits,  409-435 

bed-rock,  420 

cavity  fillings,  420,  424-428 

classification  of,  419-422 
table  of,  421 

contact-metamorphic,   418,   421,   431- 
432 

contemporaneous,  420,  422-424 

disintegration,  420,  421,  432-435 

disseminated,  428,  429 

enrichment  of,  435-435D 

fissure  veins,  425-428 

igneous,  420,  422-423 

mechanical   concentrations,   421,   433- 
435 

metamorphosed,  422,  435D 

of  high  temperature  and  pressure,  418 

of  Leadville,  Colo.,  430 

of  moderate  depth,  418,  419 

of  shallow  depth,  418,  419 

origin  of,  415-419 

oxidized  zone  of,  435,  435A,  435B 

physical  conditions  affecting,  417,  418 


INDEX 


465 


Ore  deposits,  placer,  420,  421,  433-435 

primary,  420 

replacement,  421,  428-431 

residual  concentrations,  421,  435 

secondary,  420,  421,  432-435 

sedimentary,  420,  423-424 

subsequent,  420,  424-432 

surface  alteration  of,  416,  435-435D 
Organic  decay,  25,  171-174,  175 

deposits,  112,  190-193,  272,  279,  439 
Organisms,  geologic  work  of,  171-193 

geologic  work  of,  constructive,  175-193 

geologic  work  of,  destructive,  171-174 
Orogeny,   241,   336,   373-408;      see  also 
mountains 

experimental,  383 

forces  of,  382-388 
Orogeny,  heat  in,  393-396 
Orthoclase,  25,  325,  441,  442 
Ouachita  Province,  267,*  269 
Outcrops,  297-299 
Outwash  plains,  glacial,  149 
Overfolds,  387,  392 
Overlaps,  294,  309 
Ox-bows,  66 

Oxidation  process  in  ore  deposits,  435B 
Oxide  minerals,  410,  411,  450-451 
Oxides,  hydrated,  340,  437 

in  igneous  rocks,  323 

in  magmas,  323 

in  sedimentaries,  437 

metallic,  437 
Oxidized  zone  of  ore  deposits,  435,  435A, 

435B 
Oxygen,  160 

in  atmosphere,  9 

in  life,  113,  175 

in  sea  water,  92 

work  of,  in  weathering,  25 
Ozark  Plateaus,  267,*  269 

Pacific  Border  Province,  267,*  269 

Mountain  System,  267,*  268,  269 

Ocean,  atolls  of,  186 

subsidence  of,  187,  188 
Pahoehoe  lavas,  205,  206* 
"Paint-pots,"  in  springs,  230 
Paleogeography,  5 
Paleontology,  definition  of,  4,  5 
"Parasitic"  cones,  in  volcanoes,  210 
Peat,  82,  175-180 

amount  of,  in  U.  S.,  180 

bogs,  human  remains  in,  180 

filling  of  lakes  by,  176,  177* 

in  deltas,  180 

properties  of,  179 

relation  of,  to  coal,  180 

uses  of,  179 
Pebbles,  40,  48,  273,  281,  282 

in  glacial  deposits,  145,  149 
Pegmatites,  328 
Peneplains,  70-71,  402-403 

marine,  102 


Peneplains,  upraised,  72,  76 
Pennsylvania,  folded  strata  in,  386 
Perched  blocks,  146 
Peridotite,  329,  332,  349,  447 

porphyry,  330 
Petrology,  4,  330 
Phacolites,  317 
Phenocrysts,  327 
Phonolite,  333 
Phosphates,  174 
Phosphorescence,  116 
Phosphorus,  in  life,  113 
Phyllites,  346,  347 
Physical  Geology,  definition  of,  5 
Physiographic  provinces  of  U.  S.,  267- 

269 

Physiography,  definition  of,  4 
Piedmont  glaciers,  124,  126,  127* 

Province  of  U.  S.,  267,*  268 
Pisolite,  292,*  293 
Pitchstone,  209,  329,  333 
Placer  gold  of  Alaska,  434 
of  Australia,  434 
of  California,  434 
of  Klondyke,  434 

mining,  44,  433-435 

ore  deposits,  420,  421,  433-435 
Plagioclase,  441,  442 
Plains,  266 

alluvial,  58 

of  marine  denudation,  101 

river,  66 

Planation,  lateral,  by  rivers,  67 
Planetesimal  hypothesis,  264 
Planets,  258 
Plankton,  113,  116 
Plants,  chemical  work  of,  171-173 

composition  of,  175 

geologic  work  of,  171-174,  175-182 

in  oceans,  114 

land,  in  sea,  113 

mechanical  work  of,  173-174 
Plateau  region,  fault-line  scarps  in,  370 
Plateau-forming    movements,     375-377, 

404 
Plateaus,  241,  266,  374,  375 

dissected,  376 

ice,  126 

lava,  221 

submarine,  241 

Platforms,  submarine,  63,  188,  189,  190 
Platinum,  332,  413,  422,  423,  434 

of  Ural  Mts.,  Russia,  435 
Pleistocene,  warping  of  Great  Lakes  in, 

240 

Plucking,  by  glaciers,  134,  138,  139 
Plugs,  390 

Polar  current,  93,*  94 
Ponds,  60,  109 
Porosity  of  rocks,  154,  155,  158 

of  soil,  155 

Porphyry,  326,  327,  328,  332-333,  356, 
441 


466 


INDEX 


Positive  elements,  lands  as,  243 

movements,    in   lithosphere,    236-238, 

240-241,  242,  386,  395,  396 
Potash,  in  igneous  rocks,  323 

in  life,  25 
Potassium  oxide,  201 

sulphate,  46,  91 
Pot-holes,  57,  58* 
Pressure,  and  metamorphism,  336 

effect  of,  in  ore  deposition,  417 

in  rocks,  279 

Primary  ore  deposits,  420 
Provinces,  physiographic,  of  U.  S.,  267- 

269 

Pudding-stone,  282 
Pumice,  207,  329,  333 
Pyramid  Lake,  87 
Pyrite,  174,  452 

gold-bearing,  412 

Pyroxene,  325,  329,  331,  332,  349,  353, 
437 

description  of,  447 

Quarries,  jointing  in,  356 

Quartz,  28,  273,  274,  281,  325,  328,  329, 

330,  331,  332,   333,  340,   346,  353, 

437,  448-449 
-diorite,  333 
Quartzite,  279,*  340,  345,  346,  347,  352, 

449 

"Quartz"  veins,  427 
Quicksilver,  434 
of  California,  419 

Radio-activity,  223,  264 

Radiolarians,  292 

Radium,  413 

Rain  and  running  water,  31-78 

-drop  impressions,  285 

wash,  32,  33,*  35,  36 
Rainfall,  31-32 
Ranges,  see  mountains 
Ravines,  32,  37,  53 
Red  clay,  of  ocean  bottom,  115,  116 
Reef-corals,  182 
Reefs,  barrier,  185 

origin  of,  186-190 

coral,  183-190 

sand,  107* 

Regolith  =  result  of  erosion,  32 
Reid,  H.  F.,  246,  253,  256,  257 
Rejuvenated  mountains,  403 

rivers,  54,  77 
Relic  lakes,  80,  87 
Replacement,  418 

ore  deposits,  421,  428-431 
Residual  bowlders,  28,*  29,*  30 

concentrations,  ore  deposits,  421,  435 

enrichment,  in  ore  deposits,  435B 
Residuals,  71 
Rhine  Valley,  225,  366 
Rhodochrosite,  413 
Rhone  glacier,  126,  138* 


Rhyolite,  333 

Ria  shore-lines,  105 

Ribbon  structure,  in  veins,  426 

Ridges,  parallel,  407 

synclinal,  404 
Rift  Valley,  East  Africa,  225,  241,  364 

366,  370 

Rifts,  in  rocks,  354,  358 
Rill-marks,  289 
Rills,  32 

Ripple-marks,  14,  15,  288-289 
Rivers,  31,  37-78 

accordant,  54 

aggrading,  59 

antecedent,  76,  77,  240 

burden  of,  40,  41-49 

channels,  below  sea  level,  70 

consequent,  75,  76* 

constructive  work  of,  57-68 

currents,  38,  40,  42 

deposits  of,  58,  68 

destructive  work  of,  38-57 

dismembered,  72 

distributaries  of,  60 

drowned,  71,*  72 

entrenched,  74 

flats  of,  58 

glacial,  135 

graded,  48 

gradient  of,  37,  38* 

history  of,  69-78 

ice  of,  119 

master  streams,  76,  77,  404 

meanders  in,  64,  65,*  66,*  74,  75 

plains,  work  on,  66 

rejuvenated,  54,  73,  77 

salts  in,  45,  161 

solution  materials  in,  45 

subglacial,  135,  137* 

subsequent,  75,  76* 

superimposed,  77 

swiftness  in,  law  of,  40 

terraces  of,  19,  67-68,  72,  73* 

traction  in,  45 

transportation  by,  40,  41-49 

trenching  of,  38,  49 

tributaries  of,  54 

underground,  164 

valleys,  49-57,  154* 
Robinson,  H.  H.,  301 
Roches  moutonnees,  140 
Rock-flowage,  zone  of,  338 

-glaciers,  118,  119,*  170 

-minerals,  438-452 

-pillars,  36* 

-salt,  284,  437,  449 

-sheets,  thrusting  of,  in  Alps,  391,*  392 

-slides,  23 

-streams,  170 

-trains,  118 
Rocking  stones,  146 

Rocks,    see    also    igneous,    sedimentary, 
metamorphic 


INDEX 


467 


Rocks,  arenaceous,  281 

argillaceous,  281 

buckling  in,  21 

calcareous,  281 

cementation  in,  164,  165,  172,  279,  338, 
339 

classification  of,  269 

colors  of,  in  deserts,  172 

columnar  structure  in,  357 

continental,  344 

contraction  in,  21 

creep  of,  32,  118,  170 

decay  of,  12 

decomposition  of,  25 

definition  of,  269 

dip  of,  299-300 

disintegration  of,  13,  21,  23,*  25 

effect  of  climate  on,  20-23 

effusive,  313 

exfoliation  in,  21,  22* 

expansion  in,  21 

extrusive,  313,  320-322 

glaciated,  98,  140,  141* 

glassy,  333-334 

in  solution,  45 

intrusive,  313-320 

jointed,  41 

matrix  of,  327 

moved  by  ice,  120 

porosity  of,  154,  155,  158 

primary,  312 

rifts  in,  354,  358 

schistose,  340 

spouting,  98 

stratified,  355,  364;  see  also  strata 

strike  of,  299-300 

study  of,  330 

texture  of,  350 

volcanic,  313 

weathering  in,  20-23 

Rocky    Mountain     System,    267,*    268, 
269 

Mts.,  322,  367,  373,  386,  390,  407 
Run-off  of  rainfall,  31-32 

St.  Peter  sandstone,  water  in,  159 
Salinas,  84 

Salisbury,  R.  D.,  295 
Salt,  173 

lakes,  83,*  84-89,  169 
detached,  86 
history  of,  87 

rock-,  284,  437,  449 
Saltation  in  river  transportation,  45 
Salton  Sea,  88 
Salts,  alum,  234 

as  aid  in  settling  muds,  42 

deposition  of,  21 

in  desert  deposits,  84,  169 

in  igneous  rocks,  323 

in  ocean  water,  91 

in  rivers,  45,  161 
San  Andreas  Rift,  244*.  245,  354 


San  Francisco  earthquake,  244-246,  247,* 

248,  249* 

San  Francisco  Mt.,  210,  223 
Sand,  28,  29,  40,  48,  106,  107,  112,  273, 
280 

-blasting,  13 

-dunes,  288,  290 

porosity  of,  155 

-reef,  barrier,  107* 

-storms,  14 

Sandstone,  162,  280,  281,  282-283,  289, 
293,  309,  449 

bedded,  273* 

calcareous,  164 

metamorphosed,  345,  352 

porosity  of,  155,  157 
Saratoga  Springs,  N.  Y.,  157,  166 
Sargasso  Sea,  93 
Scandinavia,  as  rising  land,  238 
Schist,  340,  345,  346,  347,  348-349,  389 
Schistose  texture  of  rocks,  340 
Scoria,  volcanic,  208,  329 
Scour,  by  currents,  188 

tidal,  96,  109 
Sea-caves,  98,  236* 

-cliffs,  99,  100,  101 

-level,  90,  91 

-terraces,  98,  99 
Seals,  in  lakes,  87 
Seas,  characters  of,  112 

deltas  in,  63 

epeiric,  111-112 

epicontinental,  90 

mediterraneans,  112 

shallow,  life  of,  112-113 

shelf,  90 

tides  in,  96 
Seaweeds,  112 

Secondary  enrichment  zone,  435A,  435C 
precipitation  of  sulphides  in,  435C 

ore  deposits,  420,  421,  432-435 

sulphides,  435C 
Sedimentary  ore  deposits,  420,  423-424 

rocks,    271-311;     see    also    sediments, 
deposits,  beds,  and  strata 
colors  of,  172 
kinds  of,  280-284 

Sedimentation,   271-311;    see  also  sedi- 
ments 
Sediments,  consolidation  of,  279 

fresh-water,  239 

in  geosynclines,  306 

in  lakes,  80 

in  solution,  41 

in  suspension,  41,  42 

marine,  92,  239 

origin  of,  345 

rate  of  settling  of,  42 

source  of,  32 

thickness  of,  294,  295,  379 
Seepage,  of  water,  156 
Seismic  belts,  249-252 
Seismograms,  253,  254 


468 


INDEX 


Seismographs,  253 

Seismology,  243 

Selenite,  443 

Selvage,  in  ores,  426* 

Semi-arid  regions,  21,  31 

Septaria,  292 

Seracs,  133 

Sericite,  442 

Serpentine,  340,  346,  349,  437,  447 

description  of,  449 
Serpulae,  191 
Shale,  280,  281,  283,  293,  309,  355 

bedded,  273* 

metamorphosed,  345,  352 

water  in,  155,  157 
Shallow-water  region,  111 
Shallow  waters,  characters  of,  112 
Shearing,  in  metamorphism,  337 
Sheets,  356 

extrusive,  320,  322,*  332 

intrusive,  315,  318,  322,*  327,  328,  332, 

357,  390,  397 
Shelf  seas,  90 
Shell  limestone,  190,  192 
Shells,  nature  of,  190 
Shingle,  106 
Shoals,  110 

Shock,  of  earthquakes,  effect  of,  246 
Shore  platforms,  99,  101 
Shore-lines,  ancient,  294 

elevated,  101 

ria,  105 

Shores,  work  of  waves  along,  97 
Shove,  of  faults,  247,*  363,  364 
Shrinkage  of  earth,  222,  223,  260,  394,  395 
Siderite,  181,  284,  412,  413,  437 

description  of,  449 

Sierra-Cascade  Mountains,  267,*  269 
Sierra  Nevadas,  382 
Sierras,  definition  of,  400 
Silica,  46,  165,  169,  201,  231,  233,  280, 
292,  323,  328,  347,  353 

in  life,  113 
Silicate  minerals,  410 
Silicates,  338,  340 

hydrated,  438 
Siliceous  sinter,  233 
Sills,  315 
Silt,  28,  48,  274,  280 

metamorphosed,  345 
Silver,  333,  412,  414,  450 

minerals,  410,  411,  435C 

native,  412,  435C,  450 

of  Coeur  d'Alene  district,  Idaho,  430 

of  Leadville,  Colo.,  430 

Spring,  Fla.,  164 

sulphide,  451 
Sink-holes,  163 
Sinter,  calcareous,  168 

siliceous,  233 
Slate,  342-343,  345,  346,  347-348,  358 

439 
Slickensides,  359 


Slide-rock,  23 

Slip,  of  faults,  362,  363* 

Slopes,  slip-off,  74,  75* 

undercut,  74,  75* 
Smithsonite,  412,  435C 
Snake  River,  lava  fields,  221 
Snow  fields,  120-123 

-line,  120 
Soapstone,  450 
Soda,  in  igneous  rocks,  323 
Sodium  carbonate,  86,  169 

chloride,  46,  85,  91,  169 

oxide,  201 

sulphate,  46,  85,  169 
Soil,  alluvial,  27* 

colluvial,  27* 

colors  of,  29,  173 

creep  of,  32,  118,  170 

effect  of  volcanic  dust  on,  204 

-formation,  20-30 

ice  in,  118 

kinds  of,  28 

mantle,  20,  32 

movements  of,  27,  32 

porosity  of,  155 

sub-,  27 

-waste,  33 
Solfataras,  226,  227 
Solution,  160-161 

materials,  in  rivers,  45 

volume  of  matter  removed  by,  161 
Solutions  in  metamorphism,  337 
Sonora  earthquake,  248 
Sounds,  108 
South  Georgia,  344 
Specific  gravity,  in  transportation,  44 
Sphagnum  moss,  177 
Sphalerite,  412 
Spherulites,  334 
Spits,  108-110,  290 
Sponges,  292 
Spouting  rocks,  98 

Springs,  31,  153,  154,  156-159,  164,  228- 
233,  358 

boiling,  228-233 

carbonated,  228 

deposits  by,  165,  167,  168 

fissure,  157,  228 

hot-,  166,  226,  228-234 

mineral,  157 

volcanic,  205 

warm,  157,  228 

diatoms  in,  180 

Spurs,  in  valleys,  51,  74,  142,  143* 
Stacks,  101 
Staghorn  corals,  182 
Stalactites,  166,  167* 
Stalagmites,  166,  167* 
Staurolites,  340 
Steatite,  450 

Stocks,  316,  319,  320,  322,*  327,  328,  331, 
332,  374,  390,  397 

contact  zones  in,  351 


INDEX 


469 


Stony  corals,  182 
Storm  waves,  96 
Storms,  11 

sand-,  14 
Strand-lines,  causes  of  change  in,  236 

elevated,  237 

Strata,  271,  see  also  beds,   sediments,  and 
sedimentary  rocks 

deformation  of,  296-306,  3V^ 

thickness  of,  294,  295,  379 
Stratification,  113,  271-274 

in  river  deposits,  68 
Stratified  rocks,  faults  in,  364 

joints  in,  355 
Streams,  see  also  rivers 

accordant,  54 

aggrading,  59 

erosion  of,  33,  38-41 

master,  76,  77,  404 

regulation  of,  by  forests,  34 
Stride,  glacial,  140,  141,*  144 
Strike,  of  faults,  359 

of  rocks,  299-300 

of  veins,  427 
Strike-faults,  364-365 

-joints,  356 

-slip,  of  faults,  363 
Structural  Geology,  definition  of,  5 
Structure,  and  topography,  405 

fissile,  283 
Subglacial  material,  134 

streams,  135,  137* 
Submarine  plateaus,  241 

platforms,  63,  188,  189,  190 

ridges,  volcanoes  on,  219 
Submergence,  106,  308-309 

conglomerates  of,  309 
Subsequent  ore  deposits,  420,  424-432 

rivers,  75,  76* 
Subsidence,  187,  188,  238-243,  244,  380 

and  sedimentation,  239 

of  Atlantic  border,  238 

theory,  of  origin  of  reefs,  187 
Suess,  E.,  387 
Sulphate  minerals,  410,  411 
Sulphates,  174 

ferric,  435A 

in  sea  water,  174 

in  sedimentaries,  437 
Sulphide  minerals,  410,  411 
Sulphides,  precipitation  of,  in  secondary 
enrichment  zone,  435C 

secondary,  435C 
Sulphur,  169,  203,  228,  234 
Sun,  258 

as  source  of  earth's  exterior  energy,  149 

-cracks,  286 

effect  of,  on  ocean,  92 

tidal  effect  of,  on  earth,  94 
Superglacial  material,  134 
Superior  Upland,  of  U.  S.,  267,*  268 
Superposition,  296 
Surf,  96 


Swamps,  58,  82,  154,*  178 

reclamation  of,  180 

southern,  177 

tropical,  178 
Syenite,  329,  331,  333 

-porphyry,  329 
Synclines,  297,  298,*  301,  302,  303,  304 

and  erosion,  404-407 
Synclinoria,  306,  396 

Table  rocks,  103 
Talc,  340,  437,  450 

-schist,  346,  349 
Talus,  23,  24,*  118,  185 

trains,  170 

Tangential  pressure,  in  orogeny,  382 
Temperate  waters,  life  in,  113 
Temperature,  effect  of,  in  ore  deposition, 
417,  418 
on  rock,  20-23 

isogeotherms,  262 
Tenor  of  ore,  414 
Tension,  and  faulting,  370-372 
Terraces,  lake,  87,*  88 

river,  19,  67-68,  72,  73* 

wave-built,  100 

wave-cut,  98,  99 

well,  154* 

Terrigenous  deposits,  112,  278 
Texture  of  rocks,  change  in,  350 
Thalwegs,  underground,  154,*  156 
Thermal  equilibrium,  395,  396 
Thorium,  264 
Throw  of  faults,  363 

Thrust-faults,  366-367,  388,  389,  390-393 
Thrust-plane,  367 
Thrusts  in  orogeny,  386-388 
Tidal  bores,  96 

currents,  94,  105,  110 

flats,  108 

marshes,  104,*  105 

retardation,  259 

scour,  96,  109 

time,  94 

waves,  caused  by  earthquakes,  252 
Tides,  94-96 

as  records  of  earthquakes,  252 

effect  of,  on  earth,  259 

height  of,  94,  95,*  96 
Tills,  glacial,  145,  147,  282 
Tilting  of  lakes,  240 
Time,  effect  of,  on  rock  consolidation,  280 

element  of,  in  geology,  5 

geologic,  length  of,  7,  47 

in  relation  to  metamorphism,  344 
Tin  of  Cornwall,  England,  419,  434 
Titanium,  423 
Topographic  maturity,  69 

youth,  57,  69,  81 
Topography,  and  structure,  405 

development  of,  in  inclined  beds,  406,* 

407 
Topset  beds,  in  deltas,  277,  295 


470 


INDEX 


Tourmaline,  351 

Trachyte,  333 

Tracks,  of  animals,  in  rocks,  286 

submarine,  287 
Trade  winds,  11,  93 
Transportation,  38 

by  rivers,  40,  41-49 

by  winds,  12,  13 

glacial,  138,  144 

tidal,  96 
Trap,  316,  332 
Travertine,  167,  233 
Tree-ferns,  124 
Trenching  of  rivers,  38,  49 
Triassic,  of  Connecticut,  377-378 
Tripolite,  181 

Tropical  waters,  life  in,  113 
Troughs,  366 
Tsunamis,  252 

Tufa,  calcareous,  85,*  86,  168,  233 
Tuffs,  submarine,  321 

volcanic,  213,  214,  321,  329,  346,  347, 

390 

Tundra,  177 
Tunnels,  heat  in,  262 

joints  in,  356 
Tupungato,  volcano,  195 
Turfs,  peat  in  form  of,  179 

Ulmic  acid,  171 
Unconformities,  306-311 

angular,  307,*  308,*  310 

classification  of,  310 

disconformities,  311 

nonconformities,  311    ' 
Underflow,  154,*  155 
Underfolds,  387 

Underground  water,  31,  153-170 
Undertow,  97,  107 
Uniformitarianism,  7,  8 
United  States,  rate  of  rock  removal  in,  47 
Universe,  259 

Upheaval  in  lands,  236-241 
Uranium,  223,  264,  395 

Vadose  water,  228,  236 
Valley  glaciers,  124,  125,*  135* 
Valleys,  49-57 

anticlinal,  404 

drowned,  104,  239 

glacial,  141-142,  148 

hanging,  141,  142,*  143* 

immature,  51 

incised,  73 

longitudinal,  47 

mature,  51 

river,  49-57,  154* 

spurs  in,  51,  74,  142,  143* 

young,  50,*  51 
Valparaiso  earthquake,  252 
Van  Hise,  C.  R.,  339 
Vapors,  in  contact  metamorphism,  350 
Vaughan,  T.  W.,  192 


Vaughan,  T.  W.,  on  coral  islands,  190 

Vegetable  mold,  171,  172 

Vegetation,  change  of,  into  peat,  176-177 

effect  of,  on  erosion,  33,  34* 

volcanic  dust  on,  204 
Veins,  crustification  in,  426 

dimensions  of,  427 

dip  of,  427 

fissure,  425-428 

foot-wall  of,  427 

gold,  in  California,  427 

gold-quartz,  425 

hanging-wall  of,  427 

horses  in,  427 

replacement,  429 

ribbon  structure  in,  426 

strike  of,  427 
Velocity,  of  river  currents,  40,  42 

of  waves,  97 
"Verde  antique,"  350 
Vesuvius,  196,*  199,  212,  214,*  216 
Vitrophyre,  329 
Volcanic  activity,  390 

problems  of,  222-226 
regions  of,  166 

agglomerate,  213,  215 

ashes,  204,  209,  213,*  321,  329,  343 

belts,  250 

bombs,  204,  208,  321,  329 

breccias,  213,  214,  321,  329,  390 

cones,  209,  210 

dust,  19,  203,  204,  321 

eruptions,  194,  197-200 
explosive,  197,  321 
fissure,  221 
intermediate,  198 
quiet,  199,  202,  209,  320 
submarine,  220 

islands,   116,   185,  187,   188,  189,  219, 
220,  265 

lapilli,  204,  208,  209,  321 

necks,  215,  318,  319,  322,*  328,  332, 
358,  374 

scoria,  208,  329 

tuffs,  272 
Volcanoes,  194-221,  358,  374,  376 

age  of,  216 

cone-in-crater  structure  in,  212 

definition  of,  194 

dissection  of,  214 

distribution  of,  217-222 

explosive,  197,  202,  321 

extinct,  216 

form  of,  195 

gases  in,  197,  202-203 

girdle  of,  around  Pacific,  217-219 

mud,  214,  229,*  230 

nitrogen  in,  203 

on  submarine  ridges,  219 

origin  of,  222-226,  390 

products  of,  202-209 

rebuilt,  211 

relation  of,  to  magmas,  200 


INDEX 


471 


Volcanoes,  springs  in,  205 

water  of,  203,  224 

weathering  in,  214 
Vosges  land  mass,  387 
Vugs,  426 

Vulcanism,  see  volcanoes  and  volcanic  ac- 
tivity 

Warping,   as  cause  of  change  of  water- 
level,  190 

as  cause  of  lake  origin,  79 
of  Great  Lakes  region,  240 
of  lands,  240 
Waste  of  lands,  32,  272 
Water,  atmospheric,    solvent    action    of, 

162 

chemical  work  of,  23,  153,  159-164 
expansion  of,  118 
freezing  point  of,  132 
ground-,  153,  159-169,  233,  338,  339 

geologic  work  of,  159-169 

motions  of,  155 

situation  of,  153 
hard,  162 
"head"  of,  158 
in  metamorphism,  337 
juvenile,  166,  224-225,  416 
-level,  changes  in,  101,  190 
magmatic,  224-225,  228,  416 
meteoric,  416 
muddy,  cause  of,  32,  33 
of  volcanoes,  224 
running,  work  of,  37-78 
solvent  power  of,  23,  25,  160 
-table,  154,  156* 
underground,  31,  153-170 

mechanical  work  of,  169 
vadose,  228,  236 
vapor,  10,  328 

in  atmosphere,  9 

in  volcanoes,  203 


Waterfalls,  54-57 
Wave-built  terraces,  100 

-cut  terraces,  98,  99 

-marks,  289 
Waves,  96-97 

depth  of  penetration  of,  97 

erosion  by,  98-102 

storm,  length  of,  96 

tidal,  252 

velocity  of,  97 
Weathering,  12,  20-23,  38 

chemical  work  in,  23 

in  volcanoes,  214 

zone  of,  165,  338 
Weed,  W.  H.,  234 
Well  terraces,  154* 
Wells,  154 

artesian,  157,  158-159 

deep,  examples  of,  159 
Wind  belts,  11 

effect  of,  on  ocean,  92 
Winds,  destructive  work  of,  13 

trade,  11,  93 

transportation  by,  12,  13 

westerlies,  11,  94 
Wollastonite,  353 
Worms,  292 

calcareous,  191 
Wyandotte  Cave,  Ind.,  164 
Wyoming  Basin,  267,*  269 

Yellowstone  Lake,  81 

Park,  227-228,  231-232,  233,  390 

Zinc,  412,  413,  414 

minerals,  410 

of  Franklin  Furnace,  N.  J.,  435D 

of  Leadville,  Colo.,  430 

of  Mississippi  Valley,  416,  428 

of  Wisconsin,  428 
Zone  of  flowage,  415 


73   4 


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